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Immunohaematology and Bloodbank and
Department of Dermatology, Leiden University Medical Center, Leiden, The Netherlands
| Abstract |
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| Introduction |
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Differences in the antigenic repertoire of M. leprae recognized by patients and healthy exposed individuals has been proposed as another factor that contributes to development of disease or protection. The recent efforts of many laboratories in the identification and characterization of Ags of M. leprae (for a recent review see 6) has allowed comparative analysis of the antigenic repertoire recognized by both subject groups. However, these studies did not reveal striking differences and indicated that by and large the same set of Ags can be recognized by patients and healthy individuals (7, 8, 9, 10, 11, 12, 13). Although the complete antigenic repertoire is perhaps far from identified yet (e.g. 14), these studies suggest that the key to distinguishing protective from disease-inducing immunity does not lie in the recognition of protective or disease-inducing Ags.
In this study we have focused our attention to the role of HLA molecules that, by selecting and presenting antigenic peptides to T cells, may influence the quality of the immune response to M. leprae. Particular HLA class II alleles have been associated with development of tuberculoid or lepromatous leprosy, but generally not with development of disease per se (see for review Refs. 15 and 16). Since this issue has never been addressed within the context of the recognition of individual Ags, we have here studied the association of HLA-DR molecules with the recognition of a panel of protein Ags by T cells from patients and healthy subjects from a region where leprosy is endemic. As found previously, overall no gross differences in the antigenic repertoire recognized by both subject groups were observed. However, within the context of HLA-DR3, a remarkable difference in the recognition of one of the Ags analyzed (the secreted 30/31-kDa Ag) was found. Whereas this Ag was a dominant target Ag for HLA-DR3-positive healthy subjects, only one of the HLA-DR3-positive tuberculoid patients responded, pointing to an Ag- and HLA-class II allele-specific low responsiveness to this Ag in tuberculoid patients from this ethnic group. These findings confirm and extend our previous studies, where we found that tuberculoid patients display an HLA-DR3-associated T cell low- or nonresponsiveness to M. leprae (15) and indicate that HLA-DR-associated factors can have a profound effect on the Ag-specific T cell response to this bacterium.
| Materials and Methods |
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Fifty-seven unrelated leprosy patients originating from Surinam, South America, were studied, all of them being members of an etnic group consisting of Negroid individuals with a predominantly Caucasoid admixture. The patients were attending the Dermatology Departments of the Academic Medical Centre in Amsterdam, the Dijkzigt Hospital in Rotterdam, and the University Hospital in Paramaribo; they had been classified according to the five-group system described by Ridley and Jopling (45). The diagnosis was based on regular and careful clinical examination, review of the clinical histories, skin-slit smear bacteriology, histopathological examination of skin biopsies, and lepromin testing. The 19 healthy individuals originated from the same ethnic group. Typing for HLA class II specificities was performed with platelet absorbed sera in the two color fluorescence test as described previously (17).
Antigens
The Ags used in this study are listed in Table I
. M. leprae sonicates were
obtained from R. Rees (London, U.K.); Mycobacterium
tuberculosis sonicates from A. Kolk (Amsterdam, The Netherlands).
30/31-kDa (Ag 85 complex A, B, and C) proteins purified from culture
filtrate of M. tuberculosis were obtained from C. Abou-Zeid
(London, U.K.). Purified P32 (the A component of Ag 85 complex
proteins) was obtained from J. de Bruyn (Brussels, Belgium), and
purified MPT59 (the B component of the Ag 85 complex proteins) was
obtained from S. Nagai (Osaka, Japan). Purified recombinant proteins
through the World Health Organization/United Nations Development
Program/World Bank/World Health Organization Special Programme for
Research and Training in Tropical Diseases/Immunology of Leprosy
Special Program were obtained from J. van Embden (Bilthoven, The
Netherlands) (hsp10L, hsp65L, hsp65T, and hsp70L), and from M. Singh
(Braunschweig, Germany) (38T and hsp71T). Hsp16T was obtained from J.
Ivanyi (London, U.K.), and hsp18L from J. Watson (Auckland, New
Zealand). Escherichia coli strains carrying pEX2 containing
M. leprae DNA inserts derived from
gt 11 recombinants
(L2, L8, L14, L21, L24, L43, and 36L) were established as described
(18). Crobetagalactosidase fusion proteins were isolated as described
(19). Fusion proteins were approximately 50% pure as estimated from
protein profiles from SDS/PAGE gels stained with Coomassie brilliant
blue. Synthetic peptides were made on an ABIMED 422 synthesizer
(ABIMED, Langenfeld, Germany) using the simultaneous multiple peptide
synthesis method. The purity of the peptides was checked on reverse
phase C18 HPLC (Lichrospher, 60RP-select B 5 mm, 250 x 4 mm,
Merck, Darmstadt, Germany) and was shown to be routinely over 75%.
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Peripheral blood from 76 donors (19 healthy endemic controls with the same ethnic background (HC),3 42 tuberculoid leprosy (BT/TT) patients, and 15 lepromatous (BL/LL) leprosy patients) was collected by venapuncture using heparinized vacutainer tubes (Becton Dickinson, Mechelen, Belgium). PBMC cells (12 x 106), isolated by Ficoll-Paque (Pharmacia, Uppsala, Sweden) centrifugation, were stimulated with M. leprae and/or M. tuberculosis sonicate (110 µg/ml) for 7 days. T cell lines were subsequently generated by propagation of blast cells in IL-2 for 810 days. T cell lines were frozen at day 11 or 12. In total, 93 T cell lines were generated: 14 M. leprae-stimulated and 6 M.tuberculosis-stimulated lines from HC subjects, 38 M. leprae-stimulated and 14 M. tuberculosis-stimulated lines from BT/TT subjects, and 6 M. leprae-stimulated and 15 M. tuberculosis-stimulated lines from BL/LL subjects. From 18 individuals (1 HC, 11 BT/TT, and 6 BL/LL) M. leprae- as well as M.tuberculosis-stimulated lines were generated. Twenty-three T cell clones were generated from one DR3-positive HC by propagation of blasts in IL-2, followed by cloning by limiting dilution using a feeder mixture consisting of 106 30-Gy radiated allogeneic PBMC from at least three different individuals, 1 µg/ml of Leuko agglutinin (Pharmacia), and 110 µg/ml of purified protein derivative. Growing cultures were restimulated with the same feeder mixture if necessary and expanded in rIL-2 as described (20).
T cell proliferation assays
A total of 104 cloned T cells or T cells from T cell
lines were cultured in 96-well, flat-bottom microtiter plates together
with 5 x 104 irradiated (20 Gy) HLA-DR-matched PBMC
and stimulated with various concentrations of whole mycobacterial
sonicates or individual Ags. Sonicates were used at two different
concentrations of 1 µg/ml and 10 µg/ml. Purified Ags were used at
concentrations of 2 and 20 µg/ml. Semipurified fusion proteins were
used at a dilution of 1/50, 1/1,000, and 1/20,000. Synthetic peptides
were used at a concentration between 0.1 and 10 µg/ml. Control wells
contained either PHA (2 µg/ml) (Wellcome Diagnostics, Dartford,
U.K.), crobetagalactosidase (nonfused) proteins, or culture medium
alone. Cells were cultured in a total volume of 200 µl of IMDM,
supplemented with penicillin (100 µg/ml), streptomycin (100 U/ml),
and glutamin (20 nM) (all Life Technologies, Grand Island, NY) and 10%
normal human AB serum. After 60 h, 1 µCi [3H]TdR
was added to each well. The cells were harvested 18 h later on a
glass fiber filter using a cell harvester. The [3H]TdR
incorporation was measured by liquid scintillation counting. The
results of the responses to sonicates of M. leprae and
M.tuberculosis and to the purified proteins hsp10L, hsp16T,
hsp18L, 30T, 38T, hsp65L, hsp65T, hsp70L, and hsp70T are expressed as
stimulation indices (SI) as well as by
cpm. SI is the ratio of
[3H]TdR incorporation of Ag-stimulated cultures to that
of cultures of the same T cell line containing medium alone.
cpm is
the [3H]TdR incorporation of Ag-stimulated cultures minus
that of cultures of the same T cell line containing medium alone.
Background responses (cultures containing medium alone) varied from
10020,100 cpm (median value 300 cpm) for HC, from 10012,100 cpm
(median value 200 cpm) for BT/TT patients, and from 1003,200 cpm
(median value 300 cpm) for BL/LL patients. Evaluation of the results of
responses to semipurified fusion protein Ags is based on
[3H]TdR incorporation of Ag-stimulated cultures compared
with cultures of the same T cell line containing semipurified
crobetagalactosidase (nonfused) protein. Responses displaying both an
SI
3 and
cpm
1000 cpm were defined as a positive
response. Ten of the 20 lines from HC, 15 of the 52 lines from BT/TT
patients, and 7 of 21 lines from BL/LL patients were tested twice or
more.
Statistical analyses
Statistical differences between various groups were tested using the two-sided Fishers exact p test. In case of groups with zero cases, 0.5 was added to make calculations of odds ratios and two-sided p values possible.
| Results |
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To study the antigenic repertoire recognized by T cells from
individuals exposed to M. leprae, a panel of 93 M.
leprae- or M. tuberculosis-stimulated T cell lines was
generated derived from three subject groups: tuberculoid (BT/TT)
leprosy patients, lepromatous (BL/LL) leprosy patients, and healthy
"exposed" individuals (HC). All individuals originated from
Surinam, a leprosy-endemic area. The antigenic repertoire recognized by
these lines was examined by studying the proliferative T cell response
to a panel of 16 different mycobacterial Ags (Table I
). Table II
summarizes the responses to these Ags
by 20 T cell lines from HC, 52 T cell lines from BT/TT, and 21 T cell
lines from BL/LL subjects. In agreement with the reported M.
leprae-specific T cell low- or nonresponsiveness in BL/LL
patients, these subjects displayed low recognition frequencies (43%)
to M. leprae sonicate as compared with HC (95%) and BT/TT
subjects (81%). A high frequency of responses (87100%) to M.
tuberculosis sonicate was detected in all three subject groups.
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When responses of M. tuberculosis-stimulated lines were compared with those of M. leprae-stimulated lines, no significant differences were found in the Ag recognition profiles. Most Ags (except hsp10), however, were recognized more frequently by M. tuberculosis-stimulated lines even when generated from the same individual (data not shown), suggesting a more prominent presence of most Ags in the M. tuberculosis preparation as compared with the M. leprae sonicate.
HLA-DR phenotype and recognition of 30/31-kDa Ag
HLA-DR molecules play an important role in the induction of T cell
responses to mycobacteria since they are the main molecules that
present peptide Ag to CD4+ T cells. To examine the
influence of HLA-DR molecules on the recognition of specific Ags in
different subject groups, most T cell donors in this study (70 of 76)
were HLA typed. The most frequently observed DR alleles appeared to be
DR2, -3, and -5. Within the group of healthy individuals, a specific
and significant association of DR3 with high T cell responses to the
30/31-kDa Ag was found (two-sided p value = 0.01;
see Fig. 1
B). This association
was HLA-DR3-specific (Fig. 1
, A and C), as well
as Ag-specific, since it was not found in case of any other Ag (as an
example, hsp65 is shown in Fig. 1
, C-F) or when
responses to whole sonicate of M. leprae were analyzed (not
shown). In contrast, in BT/TT patients, a DR3-specific low T cell
responsiveness to the 30/31-kDa Ag was found (two-sided p
value = 0.005; see Fig. 1
B), whereas no clear
association was found in BL/LL.
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Thus, HLA-DR3 is associated with 30/31-kDa-specific high T cell responsiveness in healthy exposed individuals, but with 30/31-kDa specific low T cell responsiveness tuberculoid leprosy, in this population. Moreover, within the group of HLA-DR3-positive individuals, 30/31-kDa-directed T cell responsiveness is strongly reduced in tuberculoid as well as lepromatous patients, compared with healthy controls.
Mapping of epitopes on the 30/31-kDa Ag
To define the putative HLA-DR3-restricted epitope(s) on the
30/31-kDa Ag, we raised 23 DR3-restricted
Mycobacterium-specific T cell clones from a DR3-positive
healthy contact. Of these 23 clones, nine recognized the 30/31-kDa Ag,
and two recognized the 65-kDa Ag. One other clone recognized the 15-kDa
Ag (L8) (Spierings et al., manuscript in preparation). The remaining
clones did not recognize any of the 16 Ags described in Table I
and are
of as yet unknown Ag specificity. Analysis of the nine
30/31-kDa-specific clones with pools of overlapping 15-mer peptides
covering the complete Ag M. leprae Ag 85B revealed that
seven of these recognized an HLA-DR3-restricted peptide at aa positions
5165. One clone was restricted via HLA-DPw4 and recognized an epitope
covered by aa positions 131145. One clone, interestingly, did not
respond to any of these peptides derived from 85B, suggesting that it
may recognize a specific peptide on 85A or 85C members of the Ag 85
complex. Subsequent use of overlapping 10-mer peptides allowed the DR3-
and DPw4-restricted epitopes to be mapped to positions 5665
(MGRSIKVQLQ) and positions 131140 (TTYKWETFLT), respectively. A
representative experiment showing responses of two DR3-restricted
clones LB104 and LC1011 and of the DPw4-restricted clone L2D7 to these
10-mer peptides is shown in Table III
.
L2D7 did not respond to the 85A Ag P32 nor to the 85C-derived peptide
homologue that differed only at the N-terminal amino acid (T to Y, see
Table III
). The HLA-DR3 and DPw4 restriction of these T cell clones was
demonstrated by using HLA class II isotype-specific mAbs (not shown).
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| Discussion |
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In contrast to most previous studies that employed peripheral mononuclear cells, we have here used T cell lines because this may help to avoid the stimulation of background T cell proliferation by copurifying E. coli contaminants. Our main aim was to investigate the antigenic repertoire recognized by the healthy exposed population as compared with patients, in particular, in the context of the HLA-DR molecules expressed by these individuals. In analogy to previous studies in the overall population, we found large variations in the frequency by which Ags are recognized. Some Ags (like hsp10, 30/31-kDa, and hsp65) are more "immunodominant" than other Ags, while 4 of the 16 Ags were not recognized at all. These findings agree with other studies that employed peripheral mononuclear cells, T cell clones, or limiting dilution analyses (12, 13, 22). They also support the notion that some Ags play a relatively prominent role in the immune response to M. leprae and other mycobacteria. Factors that promote the Ags availability to the immune system, such as secretion from the bacterial cell, timing, and its high level expression inside infected host cells, as well as (genetic) host factors, can all determine whether an Ag will become a prominent immune target. Current evidence (8, 22, 23, 24, 25, 26, 27) indicates that hsp10 and hsp65-kDa heat shock Ags, and the secreted 30/31-kDa Ags fulfill at least one of these criteria, and the latter molecules have been proposed as potential candidate Ags in a subunit or DNA vaccines against M. tuberculosis (28, 29).
Several studies have revealed associations of the type of leprosy with the presence of certain HLA-DR molecules (reviewed in 15), and, in the population studied here, HLA-DR3 was previously reported to be associated with tuberculoid leprosy (15, 16). Here we find a significant difference between both subject groups in the recognition of the secreted 30/31-kDa Ag: HLA-DR3 was associated with high T cell responsiveness to this Ag in healthy exposed individuals, whereas, conversely, it was associated with low T cell responsiveness to this Ag in tuberculoid patients. Our previous observation that HLA-DR3 is associated with low T cell responsiveness to M. leprae in tuberculoid patients (15, 30), combined with the current study reporting an HLA-DR3-associated low responsiveness to the 30/31-kDa Ag but not to hsp65, suggests that the 30/31-kDa Ag is prominently involved in this MHC allele-specific low responsiveness.
Our study confirms other studies (7, 8, 9, 10, 11, 12, 13, 14) that, as far as can be assessed with the Ags studied, there are no Ags that are exclusively recognized by T cells from either healthy contacts or patients. This finding indicates that disease status is not mirrored by a specific repertoire antigenic in healthy contacts or tuberculoid patients. However, some Ags, in particular 30/31-kDa and hsp65, were more frequently recognized by healthy individuals, whereas others such as the 15-kDa Ag induced more frequent responses in tuberculoid patients. The preferential recognition of hsp65 and 30/31-kDa in healthy individuals (9, 12, 25) and of 15-kDa by tuberculoid patients (31) was also found in studies that employed peripheral mononuclear cells. This finding may be a reflection of the fact that development of disease progresses with a quantitative rather than a qualitative change in the antigenic repertoire. It may be that, with continuing exposure to M. leprae, the response to Ags that are dominant early in infection (32, 33), such as 30/31-kDa and hsp65, may become down-regulated, whereas responses to other Ags such as the 15-kDa Ag become more prominent. Since the 30/31-kDa and hsp65 Ag are likely to be important targets for early acquired specific immunity, individuals that develop low responsiveness to these Ags may fail to mount an effective immune response to M. leprae, which in turn may lead to disease manifestations such as tuberculoid leprosy. The low T cell responsiveness to the 30/31-kDa Ag in tuberculoid patients in the context of HLA-DR3 in this study serves as a particularly dramatic example. The processes that underlie the DR3-associated nonresponsiveness to the 30/31-kDa Ags in tuberculoid patients are currently unknown. The 30/31-kDa Ag contains an immunodominant DR3-restricted epitope that is recognized by the majority of DR3-positive healthy contacts. Further testing of the recognition of this immunodominant epitope by tuberculoid patients may reveal whether the DR3-associated low responsiveness to the 30/31-kDa Ag in these patients is indeed focused uniquely toward this epitope. This will provide a further basis to elucidate the mechanism of this HLA class II immune response gene regulation, which is associated with the development of tuberculoid leprosy in this population.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Jelle E. R. Thole at the current address: Division of Immunological and Infectious Diseases, TNO Prevention and Health, Zernikedreef 9, P.O. Box 2215, 2301 CE Leiden, The Netherlands. E-mail address: ![]()
3 Abbreviations used in this paper: HC, healthy control; SI, stimulation index; BT/TT, tuberculoid leprosy; BL/LL, lepromatous leprosy. ![]()
Received for publication July 6, 1998. Accepted for publication March 17, 1999.
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