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The Journal of Immunology, 2001, 167: 4187-4195.
Copyright © 2001 by The American Association of Immunologists

Neonatal Exposure to Antigen Primes the Immune System to Develop Responses in Various Lymphoid Organs and Promotes Bystander Regulation of Diverse T Cell Specificities1

Christopher D. Pack2, Aimee E. Cestra3, Booki Min4, Kevin L. Legge5, Lequn Li, Jacque C. Caprio-Young, J. Jeremiah Bell, Randal K. Gregg and Habib Zaghouani6

Department of Microbiology, University of Tennessee, Knoxville, TN 37996


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Neonatal exposure to Ag has always been considered suppressive for immunity. Recent investigations, however, indicated that the neonatal immune system could be guided to develop immunity. For instance, delivery of a proteolipid protein (PLP) peptide on Ig boosts the neonatal immune system to develop responses upon challenge with the PLP peptide later. Accordingly, mice given Ig-PLP at birth and challenged with the PLP peptide as adults developed proliferative T cells in the lymph node that produced IL-4 instead of the usual Th1 cytokines. However, the spleen was unresponsive unless IL-12 was provided. Herein, we wished to determine whether such a neonatal response is intrinsic to the PLP peptide or could develop with an unrelated myelin peptide as well as whether the T cell deviation is able to confer resistance to autoimmunity involving diverse T cell specificities. Accordingly, the amino acid sequence 87–99 of myelin basic protein was expressed on the same Ig backbone, and the resulting Ig-myelin basic protein chimera was tested for induction of neonatal immunity and protection against experimental allergic encephalomyelitis. Surprisingly, the results indicated that immunity developed in the lymph node and spleen, with deviation of T cells occurring in both organs. More striking, the splenic T cells produced IL-10 in addition to IL-4, providing an environment that facilitated bystander deviation of responses to unrelated epitopes and promoted protection against experimental allergic encephalomyelitis involving diverse T cell specificities. Thus, neonatal exposure to Ag can prime responses in various organs and sustain regulatory functions effective against diverse autoreactive T cells.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Medawar et al. (1) demonstrated almost half a century ago that rodents injected at birth with splenocytes from a genetically different donor were able to accept transplants from that donor as an adult. These landmark experiments suggested that neonatal T cells were somehow susceptible to the induction of tolerance. Ever since, the neonatal period has been viewed as a window during which the introduction of Ag leads to tolerance. Initially, inactivation and/or deletion of T cells were considered the leading mechanisms for Ag-induced neonatal tolerance (2, 3). However, recent investigations have demonstrated that neonatal exposure to Ag can prime, rather than inactivate or delete, T cells (4, 5, 6, 7), and both Th1 and Th2 cells develop upon this initial Ag encounter. Surprisingly, rechallenge with the same Ag later in life elicits a predominant Th2 response (8, 9, 10, 11, 12). Since this type of biased response does not support inflammatory reactions, neonatal tolerization with Ag has been applied to deviation of specific naive T cells and has proven effective in the prevention of autoimmune diseases such as experimental allergic encephalomyelitis (EAE)7 (13) and type I diabetes (14). Although the exact mechanism for the apparent selective maintenance of Th2 cells is unknown, it has been suggested that primary Th1 cells that arise upon neonatal exposure to Ag are more vulnerable to apoptosis (15). Recent investigations have envisioned the involvement of regulatory T cells to maintain such biased neonatal immunity (16). Whatever the mechanism might be, factors such as the dose (17), the form (18), the adjuvant administered in conjunction with (12), the APC presenting (5), as well as the in vivo availability of the Ag (19) were shown to control the induction of neonatal tolerance.

Previous investigations using Igs as a vehicle for peptide delivery revealed yet another bias in neonatal-induced immunity (20, 21, 22). Indeed, Ig-proteolipid protein (Ig-PLP)1 (23, 24), a chimera encompassing the PLP139–151 epitope (25), given to mice in saline on the day of birth induced an organ-specific regulation of T cells involving a deviation in the lymph node and a novel form of anergy in the spleen (20, 21, 22). Specifically, mice given Ig-PLP1 on the day of birth and challenged with PLP1 peptide at 7 wk of age developed PLP1-specific T cells in the lymph node that produced IL-4 instead of IL-2. In the spleen, the cells, although nonproliferative and unable to produce IFN-{gamma}, secreted significant amounts of IL-2. Furthermore, when supplied with IL-12 or IFN-{gamma}, these cells regained proliferative and IFN-{gamma} responsiveness. However, free PLP1 peptide given to mice on the day of birth in saline had no effect on the adult response to a challenge with PLP1 in CFA, and the animals were not protected against EAE (20, 21). This suggests that the Ig backbone contributed to development of the novel form of neonatal immunity seen with Ig-PLP1 (20, 21, 22). Furthermore, a physical link of the peptide to the Ig is required, as injection of free peptide mixed with the Ig backbone (referred to as Ig-W) had no effect on the adult response to PLP1 in CFA (21). Also, PLP1 in IFA on the day of birth, although protective against EAE, generates a response to immunization with peptide in CFA characterized by a deviated T cell response in the spleen, but the lymph node was unresponsive (20, 22). Consequently, delivery of peptide on Ig provides adjuvanticity and confers on the peptide the ability to protect against autoimmunity by a novel mechanism involving lymph node deviation and IFN-{gamma}-dependent splenic anergy (20, 21, 22).

The study presented herein explores whether this new form of tolerance is intrinsic to the PLP1 epitope or is applicable to other epitopes presented on Igs. To address this issue, the myelin basic protein (MBP) 87–99 sequence, or MBP3 (26), was incorporated into the same Ig vehicle, and the resulting Ig-MBP3 chimera was tested for induction of neonatal immunity and protection against EAE. Surprisingly, the results show that mice given Ig-MBP3 in saline on the day of birth and challenged with MBP3 in CFA at the age of 7 wk developed T cell responses in both the lymph node and spleen. T cells of both lymphoid organs were deviated and produced IL-4 instead of IL-2 or IFN-{gamma}. Furthermore, the splenic, but not the lymph node, T cells produced elevated levels of IL-10 when subjected to prolonged (72-h) peptide stimulation. Interestingly, this environment enabled unrelated autoreactive T cells to diverge into the Th2 pathway, leading to the prevention of EAE involving diverse T cell specificities.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Mice

SJL/J (H-2s) mice were purchased from Harlan Sprague Dawley (Frederick, MD) and maintained in our animal facility for the duration of experiments. For the generation of newborn mice, breeding sets of one adult male and three females were caged together. When pregnancy was visible, the females were separated and caged individually. Offspring were weaned when they reached 3 wk of age. All experimental procedures were conducted according to the guidelines of the institutional animal care committee.

Peptides

All peptides used in this study were purchased from Research Genetics (Huntsville, AL) and purified by HPLC to >90% purity. MBP3 peptide (VHFFKNIVTPRTP) encompasses an encephalitogenic epitope corresponding to aa 87–99 of MBP (26). PLP2 peptide (NTWTTCQSIAFPSK) encompasses an encephalitogenic sequence corresponding to aa 178–191 of PLP (27). PLP1 peptide (HSLGKWLGHPDKF) encompasses an encephalitogenic sequence corresponding to aa 139–151 of PLP (25). All three peptides are presented to T cells in association with I-As MHC class II molecules and induce EAE in SJL/J mice (25, 26, 27).

Ig chimeras

Ig-MBP3 is a chimera expressing MBP3 peptide that corresponds to aa 87–99 of MBP. Construction of Ig-MBP3 used the genes coding for the L and H chains of the anti-arsonate Ab, 91A3, according to the procedures described for the construction of Ig-nucleoprotein (28). In brief, the 91A3VH gene was subcloned into the EcoRI site of pUC19 plasmid and used as template DNA in PCR mutagenesis reactions to generate 91A3VH fragments carrying the MBP3 (91A3VH-MBP3) sequence in place of CDR3. The 91A3VH-MBP3 fragment was then subcloned into an expression vector (28) in front of the exons coding for the constant region of a BALB/c {gamma}2b. This plasmid was then cotransfected into the non-Ig-producing SP2/0 myeloma B cell line with an expression vector carrying the parental 91A3 L chain. Transfectants producing Ig-MBP3 were selected in the presence of geneticin and mycophenolic acid. Ig-PLP2, which encompasses aa 178–191 of PLP, was previously described (21). Ig-W, the parental Ig not encompassing any foreign peptide, has also been described (28). Large scale cultures of transfectoma cells were conducted in DMEM containing 10% iron-enriched calf serum (BioWhittaker, Walkersville, MD). Purification of Ig-MBP3, Ig-PLP2, and Ig-W was conducted on separate columns of rat anti-mouse {kappa}-chain mAb coupled to cyanogen bromide-activated Sepharose 4B (Amersham Pharmacia Biotech, Piscataway, NJ). Large scale culture was conducted with bovine serum and media that contain minimal amounts of endotoxin. Also, since the Ig chimeras were purified by affinity chromatography, the preparations were free of endotoxins.

RIA

Capture RIA was used to assess the secretion of complete Ig-MBP3 constructs from SP2/0 transfectants. Microtiter 96-well plates were coated with polyclonal rabbit anti-mouse {gamma}2b chain-specific Ab (Zymed, South San Francisco, CA; 2 µg/ml in PBS) overnight at 4°C and then blocked with 2% BSA in PBS for 1 h at room temperature. The plates were then washed three times with PBS, and 100 µl/well of supernatant from SP2/0 cells growing under selective pressure was incubated for 2 h at room temperature. After three washes with PBS, captured Ig chimeras were revealed by incubation with 1 x 105 cpm/well 125I-labeled rat anti-mouse {kappa} mAb (American Type Culture Collection, Manassas, VA) for 2 h at 37°C. The plates were then washed five times with PBS and counted using a Wallac LKB gamma counter (Rockville, MD).

Generation of T cell hybridoma

A T cell hybridoma specific for MBP3 was generated by immunizing SJL/J mice with 200 µg MBP3 peptide in 200 µl PBS/CFA (v/v) s.c. in the foot pads and at the base of each limb. After 10 days the draining lymph nodes were removed, and T cells were stimulated in vitro for two rounds in the presence of irradiated, syngenic splenocytes, 5% T-Stim supplement (Collaborative Biomedical Products, Bedford, MA), and MBP3 peptide (15 µg/ml). The culture medium used to carry out these stimulations and other T cell activation assays in this study was DMEM supplemented with 10% FCS (HyClone, Logan, UT), 0.05 mM 2-ME, 2 mM glutamine, 1 mM sodium pyruvate, and 50 µg/ml gentamicin sulfate. This MBP3-specific T cell line was then fused using polyethylene glycol 4000 (Sigma, St. Louis, MO) with the {alpha}{beta}TCR-negative thymoma BW1100 (American Type Culture Collection). Hybrids were selected by supplementing the culture medium with hypoxanthine-azaserine (Sigma). The resulting hybridomas were screened for reactivity to MBP3 peptide by testing for production of IL-2 and IFN-{gamma} in the supernatant following stimulation with irradiated (3000 rad) splenocytes in the presence of 15 µg/ml MBP3 peptide. Positive hybridomas were then cloned by limiting dilution and used to assess the presentation of MBP3 peptide from the Ig-MBP3 chimera.

Neonatal injections of tolerogen and adult immunizations with peptide

Neonatal injections of Ig chimera were performed i.p. in 100 µl saline within 24 h after birth. When the mice reached 7 wk of age they were subjected to immunization with peptide to analyze their proliferative and cytokine responses. The immunization of adult mice with either 200 µg MBP3 or a combination of 200 µg MBP3 and 100 µg PLP2 in 200 µl PBS/CFA (v/v) was conducted s.c. in the food pads and at the base of the limbs. After 10 days the mice were sacrificed to examine the elicited immune response.

Induction of EAE

EAE was induced by s.c. injection in the foot pads and at the base of the limbs with 200 µl IFA/PBS (v/v) solution containing 200 µg Mycobacterium tuberculosis H37Ra (Difco, Detroit, MI) and MBP3 peptide (200 µg), a mixture of MBP3 (200 µg) and PLP2 (100 µg), or MBP3 (200 µg) and PLP1 (100 µg) peptides. Six hours later 200 ng Bordetella pertussis toxin (List, Campbell, CA) in 100 µl PBS was given i.v.. A second injection of B. pertussis toxin was given to the mice after 48 h. Mice were then scored daily for clinical signs of EAE as follows: 0, no clinical sign; 1, loss of tail tone; 2, hindlimb weakness; 3, hindlimb paralysis; 4, forelimb paralysis; and 5, moribund or dead.

Proliferation assays

Lymph node (axillary, lateral axillary, and popliteal) and spleen cells were incubated in 96-well plates at 4 x 105 and 10 x 105 cells/100 µl/well, respectively, with 100 µl stimulator for 3 days. MBP3 was used at the optimal dose of 30 µg/ml. Therefore, the control peptides PLP1 and PLP2 were also used at 30 µg/ml. Subsequently, 1 µCi [3H]thymidine (ICN Pharmaceuticals, Costa Mesa, CA) was added per well, and culture was continued for an additional 14.5 h. The cells were then harvested and incorporated onto glass-fiber filters. [3H]Thymidine was measured using the Trace 96 program and an Inotech beta counter (Wohlen, Switzerland). A control of medium without stimulator was included for each mouse and used as background. All results presented in the figures represent counts per minute of test samples from which the background was deducted.

ELISA

Cytokine production by spleen cells was measured as previously described (20). Briefly, 10 x 105 cells/100 µl/well were incubated with 100 µl stimulator for 24 h, and cytokine production, excluding TGF-{beta}, was measured by ELISA according to the instructions provided by the manufacturer (PharMingen, San Diego, CA). TGF-{beta} was measured according to the Genzyme protocol (Cambridge, MA). Some spleen cytokine measurements were conducted at 72 h. OD405 was measured on a SpectraMAX 340 counter (Molecular Devices, Sunnyvale, CA) using SoftMAX PRO 1.2.0 software. Graded amounts of recombinant mouse IL-2, IL-4, IFN-{gamma}, IL-10 (PharMingen), and TGF-{beta} (Genzyme) were included in all experiments to construct standard curves. The concentration of cytokines in culture supernatant was estimated by extrapolation from the linear portion of the standard curve. All anti-cytokine Abs used in these studies were purchased from PharMingen, except for the pair used for detection of TGF-{beta}, which was obtained from Genzyme. Capture Abs were rat anti-mouse IL-2, JES6-1A12; rat anti-mouse IL-4, 11B11; rat anti-mouse IFN-{gamma}, R4-6A2; and rat anti-mouse IL-10, JES5-2A5. Biotinylated anti-cytokine Abs were rat anti-mouse IL-2, JES5-5H4; rat anti-mouse IL-4, BVD6-24G2; rat anti-mouse IFN-{gamma}, XMG1.2; and rat anti-mouse IL-10, JES5-16E3.

ELISPOT assay

ELISPOT assay was used to measure cytokines produced by lymph node T cells during Ag stimulation as previously described (20). Briefly, 5 x 105 cells/100 µl/well along with 100 µl stimulator were added to multiscreen-HA plates (Millipore, Bedford, MA) that had been previously coated with capture Ab. After 24 h of incubation the plates were washed and subsequently incubated with biotinylated anti-cytokine Ab overnight at 4°C. Following incubation with avidin-peroxidase (Sigma) for 1 h at 37°C, spots were visualized by adding substrate (3-amino-9-ethylcarbazole; Sigma) and then counted under a dissecting microscope. The capture and biotinylated anti-cytokine Abs used for ELISPOT were the same as those used for ELISA.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Expression of MBP3 peptide on an Ig molecule drives efficient presentation to T cells

Recent studies revealed that neonatal exposure to an Ig expressing the PLP sequence 139–151 induced, rather than suppressed, an immune response (20, 21, 22). This response was organ specific and involved Th2 deviation in the lymph node and an IFN-{gamma}-mediated splenic T cell anergy in the spleen. This investigation addresses whether this organ-specific response is intrinsic to the PLP139–151 sequence or whether delivery by an Ig is the key factor, thus allowing a similar response to develop with other peptides. In addition, we explored whether any bystander effect of the Th2 response on the response to other myelin protein epitopes might occur. To address these issues, an Ig chimera was constructed to include MBP3 within the H chain CDR3. DNA sequence analysis confirmed insertion of the nucleotide sequence for MBP3 in the correct reading frame (Fig. 1Goa). The H chain gene incorporating the MBP3 sequence was then cotransfected with the parental L chain gene into the non-Ig-secreting myeloma B cell line SP2/0 to generate cells producing complete Ig molecules. As depicted in Fig. 1Gob, supernatant from an Ig-MBP3 transfectant incubated on plates coated with anti-{gamma}2b Ab bound a rat anti-mouse {kappa} L chain mAb, indicating that the mutated H chain paired with the parental L chain and formed a complete Ig molecule. Ig-W, the parental 91A3 Ab with an intact CDR3 domain, paired as well. The better binding observed with Ig-MBP3 could be related to the site of DNA integration as well as the number of copies incorporated into the chromosomal DNA. Alternatively, peptide insertion may have influenced protein folding in such a way that the isotypic determinants are better accessible for Abs.



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  		FIGURE 1. a, Insertion of the MBP3 nucleotide sequence into the H chain variable region of the 91A3VH gene. This was done by PCR mutagenesis as previously described (28 ). Comparison of the nucleotide sequences of the wild-type 91A3VH gene to those of the chimeric 91A3VH-MBP3 indicates that the nucleotide sequence encoding full MBP3 was inserted in the correct reading frame in place of the D segment. b, Secretion of intact Ig-MBP3 chimera from transfectants. The 91A3VH-MBP3 fragment was then subcloned into an expression vector in front of the exons coding for the constant region of a BALB/c {gamma}2b. This plasmid was cotransfected into the non-Ig-producing SP2/0 myeloma B cell line with an expression vector carrying the parental 91A3 L chain. Transfectants producing Ig-MBP3 were selected in the presence of geneticin and mycophenolic acid. Detection of complete Ig-MBP3 in the supernatant from transfectoma cells was conducted by incubation of supernatant of Ig-MBP3 or Ig-W transfectants on microtiter plates coated with rabbit anti-mouse {gamma}2b chain-specific Ab and revelation of captured Ig chimeras with 125I-labeled rat anti-mouse {kappa} L chain mAb. Each bar represents the mean ± SD of triplicate determinations. *, cpm bound for the indicated sample are significantly higher (p < 0.05) than the medium control value.

 
To test Ig-MBP3 for peptide delivery and presentation to T cells, the chimera was purified by affinity chromatography from the supernatant of large scale cultures of transfectant and assayed for presentation using an MBP3-specific T cell hybridoma, designated TCH-MBP3-A7. This hybridoma line was generated by fusing MBP3-specific short term T cell lines with the {alpha}{beta}TCR-negative thymoma BW1100. As is evident in Fig. 2Go, the T cell hybridoma TCH-MBP3-A7 produced IL-2 and IFN-{gamma} upon stimulation with irradiated APCs loaded with MBP3 peptide. The negative control PLP2 peptide, corresponding to aa 178–191 of PLP and presented by I-As (27), like MBP3 peptide, did not stimulate the T cells. Similarly, Ig-MBP3 was able to induce both IL-2 and IFN-{gamma}, while Ig-PLP2, a chimera encompassing PLP2 peptide (21), did not. In addition, immunization of SJL/J mice with Ig-MBP3 in CFA induced MBP3-specific T cell responses in both the lymph node and spleen that were predominantly Th1 in nature, exhibiting MBP3-specific production of both IL-2 and IFN-{gamma} (data not shown). These results indicate that MBP3 peptide is cleaved from the Ig and presented to T cells as other peptides expressed on Igs (23, 29, 30, 31, 32).



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  		FIGURE 2. Presentation of the Ig-MBP3 chimera to specific T cells. Irradiated (3000 rad) SJL/J splenocytes (5 x 105 cells/100 µl/well) were incubated with graded amounts of Ag, and 1 h later the culture was supplemented with 5 x 104 MBP3-specific T cell hybridoma, A7. After 24 h the supernatant was harvested, and 100 µl was used for detection of IL-2 (a) and IFN-{gamma} (b). PLP2 and Ig-PLP2, Ags presented by I-As, such as MBP3 and Ig-MBP3, were used as negative controls. Each point represents the mean ± SD of triplicate determinations.

 
Neonatal injection of Ig-MBP3 confers resistance to induction of EAE by free MBP3 peptide

Neonatal tolerance was examined by injecting Ig-MBP3 in saline into SJL/J pups within 24 h after birth, challenging these mice as adults with a disease-inducing regimen of free MBP3 peptide, and then scoring daily for paralysis. As Fig. 3Go illustrates, the group of mice that received no Ig at birth (Nil group) exhibited a disease course typical for MBP3, which generally manifests a mild, monophasic, nonrelapsing/remitting disease pattern (26). In contrast, mice that were injected with Ig-MBP3 as neonates showed virtually no clinical manifestations of EAE. The control mice that received Ig-W, the parental wild-type not containing the MBP3 epitope, developed a pattern of disease resembling the Nil group.



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  		FIGURE 3. Ig-MBP3 neonatally tolerized mice resist induction of EAE by MBP3 peptide. Newborn mice (four to seven per group) were injected with 100 µg affinity-purified Ig-MBP3 (Ig-MBP3 Tol) or Ig-W (Ig-W Tol) in saline within 24 h of birth and were induced for EAE at 7 wk of age with free MBP3 peptide as described in Materials and Methods. Mice were then scored daily for signs of paralysis. For comparison purposes a group of mice that did not receive any injection (Nil) was included. The maximal SD for the groups of animals tested did not exceed 0.6.

 
Neonatal injection of Ig-MBP3 leads to normal lymph node proliferation, reduced responsiveness in the spleen, and Th2 deviation in both organs

Investigation of the proliferation and cytokine profiles of the lymph node and spleen of mice injected at birth with Ig-MBP3 and challenged at 7 wk of age with MBP3 peptide in CFA yielded results similar to those obtained from neonatal studies conducted with Ig-PLP1 (20, 21, 22). Lymph node proliferation remained unaltered in Ig-MBP3 tolerized mice vs Ig-W-tolerized mice (Fig. 4Goa). In contrast, the lymph node cytokine production of mice tolerized with Ig-MBP3 revealed a strong deviation from a Th1 to a Th2 response. Ig-MBP3-tolerized mice secreted high levels of IL-4 in response to stimulation with MBP3 peptide, while mice injected with Ig-W at birth secreted relatively none (Fig. 4Gob). In addition, while the Ig-W-tolerized mice showed weak IFN-{gamma} production in response to stimulation with MBP3 peptide, the Ig-MBP3 group did not show IFN-{gamma} production (Fig. 4Goc). These responses were specific for MBP3 peptide, since the negative control, PLP2 peptide, generated no significant response in either group. In the spleen, the Ig-MBP3-recipient group demonstrated significantly reduced proliferation (p < 0.05) in response to stimulation with MBP3 compared with mice that had been neonatally injected with Ig-W (Fig. 4God). Surprisingly, however, the cytokine profile of Ig-MBP3 mice revealed deviation to a Th2 phenotype and production of IL-4 rather than IFN-{gamma} upon stimulation with MBP3 (Fig. 4Go, e and f). These responses were also specific, as there was no detectable response to PLP2.



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  		FIGURE 4. Exposure to Ig-MBP3 on the day of birth drives T cell deviation in both lymph node and spleen. Newborn mice (eight per group) were injected i.p. within 24 h after birth with either 100 µg Ig-MBP3 or Ig-W in saline. When the mice reached 7 wk of age they were immunized with 200 µg free MBP3 peptide in 200 µl CFA/PBS (v/v) s.c. in the foot pads and at the base of the limbs. Ten days later the mice were sacrificed, and the draining lymph node (LN; 0.4 x 106 cells/well) and spleen (SP; 1 x 106 cells/well) cells were stimulated with 30 µg/ml free MBP3 or PLP2. The lymph node (a) and spleen (d) proliferation was measured by [3H]thymidine incorporation after 3 days of stimulation. Cytokine production was analyzed in the LN by ELISPOT (b and c) and in the SP by ELISA (e and f) after 24 h of stimulation. The indicated values represent the mean ± SD of eight individually tested mice. Statistical analysis (Student’s t test) was performed. *, Difference between the test group (Ig-MBP3) and the control (Ig-W) group is significant (p < 0.05).

 
Neonatal exposure to Ig-MBP3 confers resistance to the induction of EAE with multiple epitopes

Since Ig-MBP3 neonatally tolerized mice exhibited Th2 deviation in both lymph node and spleen, we hypothesized that such an environment might be able to modulate the response to a separate myelin epitope associated with EAE. To address this issue, mice were injected with either Ig-MBP3 or Ig-W at birth and at the age of 7 wk were assayed for resistance to EAE induction with multiple epitope regimens. As shown in Fig. 5Go, Ig-MBP3-tolerized mice showed a significant reduction in clinical paralytic severity when they were induced for disease with a mixture of MBP3 and PLP2 peptides. Mice that had received no Ig molecule (Nil group) during the neonatal period and those injected with the control Ig-W molecule had a normal disease pattern (Fig. 5Goa). In addition, Ig-MBP3 neonatal tolerization conferred resistance against a regimen including MBP3 and PLP1 peptides. Although Ig-MBP3-tolerized mice exhibited only a slightly reduced initial disease peak compared with both the Ig-W-injected and Nil control groups, they were completely protected from relapses (Fig. 5Gob). In contrast, both control groups suffered a severe relapse that resulted in a significant mortality rate (see Table IGo).



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  		FIGURE 5. Ig-MBP3 neonatally tolerized (Tol) mice show resistance to EAE induced with two epitopes. Newborn mice (5–10/group) were injected with 100 µg affinity-purified Ig-MBP3 or Ig-W in saline within 24 h of birth and were induced for EAE at 7 wk of age with a mixture of MBP3 and PLP2 (a) or MBP3 and PLP1 (b) peptides or with PLP2 alone (c) as described in Materials and Methods. Mice were then scored daily for signs of paralysis. A group of mice that did not receive any injection on the day of birth (Nil) was included for control purposes. The maximal SD did not exceed 0.6 for groups shown in A and C and 1 for the groups shown in B.

 

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  		Table I. Reduction of the clinical severity of EAE in Ig-MBP3-tolerized neonates1

 
To show that the deviated MBP3-reactive T cells must be activated at the same time as the naive PLP1- or PLP2-specific pathogenic T cells to achieve protection, EAE was induced in Ig-MBP3- or Ig-W-tolerized mice with PLP2 peptide, and the signs of paralysis were assessed. As illustrated in Fig. 5Goc, the two groups showed no significant difference in disease pattern. Therefore, it appears that neonatally induced MBP3-specific T cells had to be activated by the Ag during induction of disease with PLP2 to support deviation and/or modulation of PLP2-specific T cells and prevent the development of EAE.

Organ-specific regulation of IL-10 production in neonatally tolerized mice

Although both the lymph node and splenic T cells of Ig-MBP3-tolerized mice were deviated and produced IL-4, the proliferative response of the splenic cells was significantly reduced relative to that of their Ig-W counterparts. This suggested that the splenic T cells might be producing anti-proliferative cytokines such as IL-10 or TGF-{beta} (34, 35). To test this hypothesis the MBP3-specific T cells from both lymphoid organs were assayed for production of IL-10 and or TGF-{beta}. The results indicated that although after a 24-h stimulation neither TGF-{beta} nor IL-10 was detectable (not shown), at 72 h a high level of IL-10 was observed in the spleen (Fig. 6Goa). The lymph node T cells did not produce any detectable IL-10 in either group (Fig. 6Gob). Cells from the control mice tolerized with Ig-W did not secrete a detectable level of IL-10. No TGF-{beta} was observed in either group (data not shown). This IL-10 production in Ig-MBP3-tolerized mice was specific for MBP3 peptide, since in vitro stimulation with PLP2 peptide yielded no significant IL-10 production.



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  		FIGURE 6. Production of IL-10 in the spleen, but not the lymph node, of Ig-MBP3-tolerized and peptide-immunized mice. Newborn mice (eight per group) were injected within 24 h after birth with either 100 µg Ig-MBP3 or Ig-W in saline. When the mice reached 7 wk of age they were immunized with 200 µg free MBP3 peptide in 200 µl CFA/PBS (v/v) s.c. in the foot pads and at the base of the limbs. Ten days later the mice were sacrificed, and the lymph node (0.4 x 106 cells/well) and spleen (1 x 106 cells/well) cells were stimulated with 30 µg/ml free MBP3 or PLP2 for 72 h. IL-10 production was measured by ELISA in the spleen (SP) and by ELISPOT in the lymph node (LN). The indicated values represent the mean ± SD of eight individually tested mice. *, Difference in the amount of cytokine produced by the Ig-MBP3 group vs that produced by the corresponding Ig-W group is significantly different (p < 0.05).

 
Since recent studies have revealed that APCs could, under certain circumstances, produce IL-10 (36), we used purified T cells and mitomycin C-treated APCs to ascertain whether the IL-10 seen in the culture was produced by MBP3-specific T cells. To this aim, purified T cells from Ig-MBP3- and Ig-W-tolerized mice were stimulated with MBP3 peptide-loaded, mitomycin C-treated splenocytes from naive SJL/J mice. After 6 days of culture the cytokine production of both groups was analyzed. Adult splenic T cells derived from Ig-MBP3-tolerized mice demonstrated a polarized Th2 response characterized by elevated levels of both IL-4 and IL-10 (Fig. 7Go, b and c) in response to free MBP3 peptide as well as a significant decrease in the level of IFN-{gamma} secretion (Fig. 7Goa). Mice that received Ig-W as neonates mounted only a characteristic Th1 response against MBP3 peptide typified by IFN-{gamma} release (Fig. 7Goa). These responses were specific to MBP3 in each case, as stimulation with the control peptide, PLP2, yielded negligible cytokine production.



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  		FIGURE 7. T cells, not APCs, produce the splenic IL-10 in Ig-MBP3-tolerized mice. Newborn mice (four per group) were injected i.p. within 24 h after birth with either 100 µg Ig-MBP3 or Ig-W in saline. When the mice reached 7 wk of age they were immunized with 200 µg free MBP3 peptide in 200 µl CFA/PBS (v/v) s.c. in the foot pads and at the base of the limbs. Ten days later the mice were sacrificed, and the spleen was homogenized to a single-cell suspension and passed over a nylon wool column to enrich for T cells. Purified T cells (1 x 105 cells/well) were then stimulated for 6 days with APCs that were previously pulsed with either 30 µg/ml free MBP3 or PLP2 peptide and treated with mitomycin C (50 µg/ml for 30 min). Subsequently, the production of IFN-{gamma} (a), IL-4 (b), and IL-10 (c) in the supernatant was analyzed by ELISA. The indicated values represent the mean ± SD of four individually tested mice. APCs or T cells alone did not produce any significant cytokine. *, The amount of cytokine produced by the corresponding group is statistically significant (p < 0.05).

 
IL-10 produced by neonatally induced MBP3-specific T cells displays bystander function and suppresses the responsiveness of diverse T cells

In an effort to delineate the mechanism underlying the resistance to EAE induction with multiple epitopes and to assess the bystander function of MBP3-specific T cells, mice that received either Ig-MBP3 or Ig-W at birth were immunized as adults with a combination of MBP3 and PLP2 peptides in CFA. The T cell response to both peptides was then analyzed. The results indicate that the spleen proliferation of Ig-MBP3-tolerized mice was significantly reduced in response to both MBP3 and PLP2 peptide (Fig. 8Goa) compared with that of Ig-W-tolerized mice. Even more surprising, PLP2-specific T cells produced significant amounts of IL-4, and their IFN-{gamma} levels were reduced (Fig. 8Go, b and c). Further examination of IL-10 production indicated that these cells were producing elevated levels (Fig. 8God). The mice tolerized with Ig-W instead of Ig-MBP3 had significant proliferation and IFN-{gamma} production in response to both peptides, with neither IL-4 nor IL-10 observed.



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  		FIGURE 8. Neonatal injection of Ig-MBP3 induces bystander suppression of diverse T cell specificities. Newborn mice (eight per group) were injected i.p. within 24 h after birth with either 100 µg Ig-MBP3 or Ig-W in saline. When the mice reached 7 wk of age they were immunized with a combination of 200 µg MBP3 and 100 µg PLP2 peptides in 200 µl CFA/PBS (v/v) s.c. in the foot pads and at the base of the limbs. Ten days later the mice were sacrificed, and the spleen (1 x 106 cells/well) cells were stimulated with 30 µg/ml MBP3, PLP2, or PLP1. Proliferation (a) was measured after 3 days of stimulation, the cytokines IL-4 (b) and IFN-{gamma} (c) were measured after 24 h of stimulation, and IL-10 (d) was measured after 72 h of stimulation. The indicated values represent the mean ± SD of eight individually tested mice. *, Data are statistically significant (p < 0.05).

 
To evaluate the contribution of IL-10 to the resistance against multiple epitope-induced EAE, mice neonatally tolerized with Ig-MBP3 were subjected to induction of EAE with a mixture of MBP3 and PLP2 peptides and given anti-IL-10 mAb. As shown in Fig. 9Go, in vivo neutralization of IL-10 by anti-IL-10 Ab restored the severity of EAE to a level comparable to that obtained in the susceptible mice neonatally injected with Ig-W. The control group injected with rat IgG instead of anti-IL-10 mAb, like the Ig-MBP3 neonatally tolerized mice not given anti-IL-10 during disease induction, failed to restore the severity of paralysis. These results indicate that splenic IL-10 produced by MBP3-specific T cells probably contributes to bystander suppression of newly migrant, MBP3-reactive, as well as unrelated T cells and prevents EAE even when diverse T cell specificities are involved.



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  		FIGURE 9. Requirement of splenic IL-10 for bystander suppression of EAE involving diverse T cell specificities. Newborn mice were injected within 24 h of birth with 100 µg Ig-MBP3 in saline. When they reached 7 wk of age, a group of four mice was injected i.p. with 1 mg/mouse affinity-purified JES5-2A5 anti-IL-10 Ab in 1 ml PBS. A second group of four mice was injected with 1 mg/mouse of rat IgG in 1 ml PBS to serve as a control. On the next day EAE was induced in all mice with a combination of free MBP3 and PLP2 peptides as described in Materials and Methods. Five days after disease induction the mice were given a second injection of 1 mg/mouse of JES5-2A5 or rat IgG. The mice were scored daily for signs of paralysis. For comparison purposes the clinical scores of a group of mice that was injected with either Ig-MBP3 or Ig-W at birth, but did not receive any Ab treatment, were included. The maximal SD for the groups of animals did not exceed 0.5. Tol, Tolerized.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
In recent years it has become clear that neonatal exposure to Ag primes, rather than ablates, immunity. However, neonatal-induced responses in most cases evolve to tolerate the Ag and do not mediate the inflammatory reactions that usually develop upon immunization of animals that have not been exposed to the Ag during the neonatal stage (8, 9, 13, 14). Moreover, in addition to these qualitative differences, neonatal exposure to Ag seems to drive an organ-specific regulation of T cells that supports the development of responses in the spleen, but leaves the lymph nodes unresponsive (12). However, if the antigenic peptide is given to newborns on an Ig, T cell responses attempt to develop in the spleen upon rechallenge with Ag later in life (20, 21, 22). Indeed, SJL/J mice given Ig-PLP1 on the day of birth and challenged with PLP1 peptide as adults generated lymph node T cells, but the development of splenic response required exogenous IL-12 (20). Specifically, the lymph node T cells were deviated and produced IL-4 instead of IL-2, and the splenic cells, although nonproliferative and unable to produce IFN-{gamma}, still secreted significant amounts of IL-2 (20). Exogenous IL-12 or IFN-{gamma} restored splenic proliferation both in vitro and in vivo. In the present studies using an Ig chimera incorporating the encephalitogenic MBP87–99 amino acid sequence (referred to as MBP3), we found that neonatal exposure to Ig-MBP3 promoted immune responses in both the lymph node and spleen. However, splenic T cells, unlike the Ig-PLP1 system, were proliferative and produced IL-4, thereby extending deviation to both lymphoid organs. As the deviation was broadened to both the lymph nodes and spleen, we sought to test the T cells for bystander modulation of neighboring cells and suppression of disease involving diverse T cell specificities. Indeed, mice tolerized with Ig-MBP3 at birth exhibited significantly reduced disease severity when they were induced for EAE with a combination of MBP3 and PLP2 peptides or MBP3 and PLP1 peptides (Fig. 5Go, a and b, and Table IGo). Also, as resistance to disease was not present when Ig-MBP3 neonatally tolerized mice were induced for EAE with PLP2 peptide, we concluded that neonatally deviated MBP3-specific T cells had to be stimulated during disease induction for bystander T cell modulation to occur (Fig. 5Goc). Furthermore, to determine how Ig-MBP3-induced Th2 cells were capable of regulating unrelated pathogenic cells and modulating the disease, two types of analyses were performed. On the one hand, we tested the neonatally induced MBP3-specific Th2 cells for production of IL-10, a cytokine that has previously been shown to mediate bystander suppression (37, 38) and exert pleiotropic, down-regulatory effects on ongoing Th1 immune responses (for review, see Ref. 34). On the other hand, we analyzed the proliferative and cytokine responses to PLP2 peptide upon immunization of Ig-MBP3 neonatally tolerized mice with the MBP3 and PLP2 peptide mixture. Splenic MBP3-specific T cells that arise in Ig-MBP3 neonatally tolerized mice upon stimulation with MBP3 peptide produced, along with IL-4, copious amounts of IL-10 when cultured in vitro for 72 rather than 24 h (Fig. 6Goa). IL-10 production was Ag specific and occurred only when the stimulator was MBP3 peptide. Neutralization of the splenic IL-10 during disease induction restored the severity of EAE, indicating that IL-10 plays a major role in the resistance to disease induction (Fig. 9Go). IL-10 and IL-4 produced by MBP3-specific T cells provide an environment suitable for the development of Th2 cells, but unfavorable for the generation of Th1 cells. This statement is supported by the observation that immunization of Ig-MBP3 neonatally tolerized mice with a combination of MBP3 and PLP2 generates weakly proliferating, deviated T cells to both peptides (Fig. 8Go). More striking, the PLP2-specific T cells also produce significant amounts of IL-10 (Fig. 8God). In addition, since neutralization of IL-10 restores disease, it is possible that during adult immunization IL-10 produced by MBP3-specific T cells exerts bystander suppression on both newly migrant, MBP3-reactive T cells that were not exposed to Ig-MBP3 during the neonatal period and PLP2-specific cells, thereby limiting Th1 development. Moreover, the simultaneous presence of IL-4 might drive these cells to differentiate into noninflammatory Th2 cells.

Overall, these observations raise two significant points. The first relates to how unrelated pathogenic T cells are deviated during immunization of Ig-MBP3 neonatally tolerized mice with a mix of peptides, and the second concerns the source of IL-10. One likely explanation for the deviation of PLP2 responses is that IL-4 and/or IL-10 from MBP3 cells guided the development of PLP2-specific cells into the Th2 pathway (39). This would assume that the cells producing IL-4/IL-10 cytokine differentiate at the same site as stimulated memory MBP3-specific T cells. As for the production of IL-10, two sources may be considered. The deviated MBP3-specific T cells secreting IL-4 could be the producer of IL-10 seen in the culture. However, the delay in such production relative to IL-4 (72 vs 24 h) suggests that the two cytokines are produced by different cells. It has been previously shown that regulatory T cells can arise in an IL-10-rich environment (40) and secrete significant levels of IL-10 (41, 42). Therefore, one could envision that the splenic cell population includes regulatory cells that function as a source of IL-10 and regulates autoimmunity (41, 42, 43, 44). This statement may garner support from our recent findings showing that exposure to Ig peptide during the neonatal stage prevents up-regulation of CD40 ligand (CD40L) expression (45), a phenomenon that could result in suboptimal expression of B7 molecules on APCs and favor the development of regulatory T cells (44). If this is the case, a question arises as to whether regulatory cells are Ag specific. If not, what is the underlying mechanism that triggers their expansion and colocalization with pathogenic T cells to modulate the disease?

The other issues that arise from these observations relate to the factors driving neonatal T cells to deviate to Th2 and home either to the lymph node only in Ig-PLP1-induced neonatal immunity or to both the lymph node and the spleen in the Ig-MBP3 system. Neonatal T cells have been shown to express quantitatively reduced levels of CD40L (46). In addition, when the exposure uses Ig-peptide instead of free peptide, CD40L expression remains at background levels (45). Consequently, upon Ag recognition on APCs, CD40-CD40L interactions would be limited, resulting in little or no production of IL-12 and a biased T cell differentiation. In recent studies we have shown that the splenic T cells in Ig-PLP1-tolerized mice lack CD40L expression and could not progress in the differentiation pathway (47). However, since the same SJL mouse strain and Ig backbone are used in the Ig-MBP3 and Ig-PLP1 systems, minimal expression of CD40L may not account for the differential pattern among T cells induced by Ig-PLP1 vs Ig-MBP3. Therefore, it is possible that a discrepancy in peptide affinity among PLP1 and MBP3 influences T cell-APC interactions, contributes to differential regulation of CD40L, and leads to an unbalanced Th2 bias between the two systems. Alternatively, as the autoimmune T cell repertoire in the SJL mouse is dominated by PLP1-specific T cells (48), it is possible that higher T cell frequency in the PLP1 system is responsible for the emergence of anergic T cells in the spleen (45).

Overall, the neonatal immune system can be guided to develop responses in both the spleen and lymph node and protect animals against autoimmunity involving diverse T cell specificities.


   Footnotes
 
1 This work was supported by startup funds (to H.Z.) from the University of Tennessee (Knoxville, TN). Back

2 Current address: Laboratory of Viral Immunology, Department of Microbiology, University of Tennessee, Knoxville, TN 37996. Back

3 Current address: Antigenics, Inc., Woburn, MA 01801. Back

4 Current address: Laboratory of Immunology, National Institute of Allergy and Infectious Disease, National Institutes of Health, Building 10, Room 11N314, Bethesda, MD 20892-1892. Back

5 Current address: Beirne B. Carter Center for Immunology Research, University of Virginia, 400 Lane Road, MR-4 Building, P.O. Box 801386, Charlottesville, VA 22908-1386. Back

6 Address correspondence and reprint requests to Dr. Habib Zaghouani at the current address: Department of Molecular Microbiology and Immunology, University of Missouri School of Medicine. M616 Medical Sciences Building, Columbia, MO 65212. E-mail address: zaghouanih{at}health.missouri.edu Back

7 Abbreviations used in this paper: EAE, experimental allergic encephalomyelitis; CD40L, CD40 ligand; MBP, myelin basic protein; PLP, proteolipid protein. Back

Received for publication May 1, 2001. Accepted for publication August 8, 2001.


   References
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 Introduction
 Materials and Methods
 Results
 Discussion
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