In This Issue

doi:10.4049/jimmunol.1390025

An Immune Road Map

Systems approaches offer the potential to interconnect large sets of data to better understand biological systems. To identify genes that are preferentially expressed in immune tissues, Giallourakis et al. (p. 5578) developed a quantitative schema that uses the National Center for Biotechnology Information human UniGene expressed sequence tag (EST) database. The authors established a database of 2,232 immune-enriched genes, or Immunogenes, and validated their EST profiling strategy using multiple other large-scale analysis systems. Characterization of each of these Immunogenes to identify its predicted functional domains and homology to other proteins revealed an enrichment of transmembrane proteins and proteins with potential enzymatic activity. Many Immunogene-encoded proteins were also predicted to function in cell metabolism. The EST profiling also identified numerous microRNAs and other forms of untranslated RNA, including long noncoding RNAs. As an example of the utility of this profiling system, the authors were able to identify PRDM15 as a transcription factor that is overexpressed in B cell lymphomas. The Immunogene database resulting from this genome-wide EST profiling will be an important resource for the continuing genetic analysis of immunity.

Let ITAM Open the Door (to Dengue)

Dengue virus (DENV) infection can lead to a variety of clinical outcomes ranging from mild dengue fever to life-threating dengue shock syndrome. The development of severe disease has been linked to Ab-dependent enhancement (ADE) of infection, which can be facilitated by FcγRs. Boonnak et al. (p. 5659) investigated the contributions of human FcγRIIa and FcγRIIb, which are expressed on DENV target cells, to ADE of DENV infection. FcγRIIa is a low affinity activating receptor that is the only Fc receptor with an ITAM motif in its cytoplasmic tail, whereas FcγRIIb is the only known inhibitory FcγR and contains an ITIM motif in its cytoplasmic tail. When assayed in the context of DENV infection, FcγRIIa supported ADE, but FcγRIIb imposed a dominant inhibitory effect on infection. The two FcRs have very similar extracellular domains and demonstrated comparable levels of binding to DENV immune complexes; however, FcγRIIa was much more effective in mediating viral internalization. Motif swapping experiments in which the FcγRIIa and FcγRIIb cytoplasmic domains were exchanged revealed that the ITAM and ITIM domains of these receptors were responsible for their differing abilities to mediate ADE of infection. Understanding the involvement of these broadly expressed Fc receptors in the severity of DENV-induced disease will have implications for strategies to combat this potentially dangerous infection.

γδ T cells, Diabetes Effectors

Referred to as innate-like T cells, γδ T cells recognize unprocessed Ag and undergo a less stringent selection process than their αβ TCR-bearing lymphocyte brethren. γδ T cells have also been implicated in the pathology of autoimmune disease, leading Markle et al. (p. 5392) to examine the role of these cells in the NOD mouse model of type I diabetes (T1D). The authors found two populations of γδ T cells, CD27^–CD44^hi and CD27⁺CD44^lo, infiltrating pancreatic islets in prediabetic NOD mice. In addition, these populations were distinguished by their secretion of IFN-γ (CD27⁺CD44^lo) and IL-17 (CD27^–CD44^hi) upon activation. Relative to other cell populations, the CD27^–CD44^hi γδ T cell subset increased the severity of disease when adoptively transferred into T and B cell–deficient NOD mice. The use of an IL-17–neutralizing Ab prevented the transfer of disease by γδ T cells. The authors also found that Tcrd deficiency, achieved through introgression on the NOD background, protected mice against the development of diabetes. The identification of CD27^– γδ T cells as effector cells in T1D, through their secretion of IL-17 and migration to the islets, indicates these cells are integral to the development of diabetes in NOD mice and suggests potential targets in the treatment of human disease.

Pathogenic Pyroptosis

Inflammatory bowel disease may result from perturbations in how commensal gut bacteria interact with the immune system. Susceptibility to severe ulcerative colitis (UC) has been linked to variations in the multidrug resistance gene (MDR1) and TLR2. In this issue, Ey et al. (p. 5676) investigated the effect of TLR2 deficiency on the development of colitis in mice lacking MDR1A. Compared with mice lacking MDR1A alone, TLR2/MDR1A double-deficient knockout mice (dKO) developed exacerbated spontaneous colitis with earlier onset and higher incidence. Colitis in dKO mice was dependent on the presence of normal commensal microbiota and involved Th1 skewing and increased early expansion of myeloid cells. Deletion of the LPS coreceptor MD-2 ameliorated disease in dKO mice, suggesting that hyperresponsiveness to LPS exacerbated colitis. Indeed, exposure of CD11b⁺ myeloid cells from dKO mice, but not from controls, to nonpathogenic Escherichia coli induced cell death that required MyD88 signaling and caspase-1 activation. This apparent pyroptosis was associated with excessive production of reactive oxygen species and resulted in the release of IL-1β, which was required for colitis exacerbation in dKO mice. In lamina propria samples from UC patients bearing TLR2 and MDR1 polymorphisms, nuclear caspase-1 expression and cell death were enhanced relative to samples from control UC patients. These data suggest that pyroptosis may be involved in severe UC and is controlled via the combined actions of TLR2 and MDR1.

MINImizing the Alternative Pathway

Inadequate control of complement activation can cause or exacerbate multiple human disorders, indicating a need for effective complement-targeted therapeutics. To inhibit the alternative pathway (AP) of complement activation, Schmidt et al. (p. 5712) engineered a molecule that is a smaller, less complex version of human factor H (FH) yet retains its key functional regions. This inhibitor, mini-FH, bound to the opsonin C3b similarly to FH but showed much higher affinity for the C3b degradation products iC3b and C3dg. Mini-FH was also shown to bind efficiently to markers of oxidative stress and host-specific surface markers. Relative to FH, mini-FH demonstrated a >10-fold increase in inhibition of the AP. In two models of paroxysmal nocturnal hemoglobinuria, mini-FH potently protected human erythrocytes from complement-mediated lysis, and was again much more effective than normal FH. In addition to revealing insights into the normal functioning of FH, the robust AP-inhibiting activity of mini-FH supports further investigation into its clinical utility.

Adapting DC Differentiation

Signaling through TANK-binding kinase 1 (TBK1) induces the production of type I IFN, and several adaptor molecules with TBK1-binding domains have been implicated in this process. To better understand this signaling pathway, Fukasaka et al. (p. 5702) generated mice deficient in two of these adaptor molecules, 5-azacytidine-induced gene 2 (AZI2) and TBK1-binding protein 1 (TBKBP1). Surprisingly, neither of these molecules was necessary for the production of type I IFN or IL-6 in macrophages or plasmacytoid dendritic cells following TLR or retinoic acid inducible gene-I–like receptor (RLR) stimulation. However, AZI2, but not TBKBP1, was important for the GM-CSF–mediated differentiation of bone marrow cells into CD11c⁺ dendritic cells (GM-DCs). Relative to wild-type cells, AZI2^−/− GM-DCs demonstrated impairments in proinflammatory cytokine production and in the ability to activate CD4⁺ T cells both in vitro and in vivo. AZI2 was necessary for cell cycle progression during GM-DC differentiation through a process requiring its TBK1-binding domain. Overexpression of AZI2 in bone marrow cells significantly enhanced differentiation of GM-DCs and their ability to stimulate CD4⁺ T cells. AZI2 therefore modulates signaling through TBK1 in a cell type-specific manner and is a key participant in GM-DC differentiation and function that might be exploited to increase the efficiency of DC vaccines.

HIV G2 Dominance

The Agence Nationale de Recherche sur le Sida Hépatites Virales (ANRS) Lipo5 vaccine is composed of five long fragments of HIV proteins, including one from Pol (P1), two from Nef (N1 and N2), and two from Gag (G1 and G2), that are linked to a palmitoyl-lysylamide moiety. Recent data demonstrated that seronegative volunteers who received the vaccine generated a CD4⁺ T cell response directed predominantly against the G2 peptide. To determine if HLA-DR binding efficiency or the frequency of naive CD4⁺ T cell precursors were behind these results, Castelli et al. (p. 5757) performed peptide binding experiments and examined CD4⁺ T cell profiles from healthy volunteers. Binding assays with the vaccine peptides revealed that four of the vaccine components, not just G2, had a broad HLA-DR molecule binding specificity. The authors created Lipo5-specific T cell lines with weekly in vitro stimulations and found that most of these lines were G2 specific. This was due to a G2-specific T cell repertoire with a frequency of 2 cells/million, contrasted with other repertoire specificities of ∼0.4 cells/million. Thus, the authors concluded that the ANRS Lipo5 vaccine stimulated such a strong G2 response because of a pre-existing naïve, Ag-specific T cell repertoire, an important concept to keep in mind when designing vaccines and evaluating efficacy.

Transplanting Alloimmunity

Transplant alloantigens can be recognized by T cells through two mechanisms: 1) the direct pathway, in which intact MHC is seen as an alloantigen on the surface of donor APCs, and 2) the indirect pathway, in which processed alloantigen is presented by recipient APCs. For direct-pathway CD8⁺ T cells to recognize donor cell alloantigen, CD4⁺ T cell help is needed. However, there has been no evidence of recipient dendritic cells simultaneously presenting donor MHC class I and processed, recipient MHC-presented alloantigen that would allow the production of this T cell help. Sivaganesh et al. (p. 5829) have now reported that after coculture with BALB/c dendritic cells (DCs), C57BL/6 DCs could present both processed and intact acquired H-2^d alloantigen. The ability of these cocultured C57BL/6 DCs to present the conformational epitope for direct-pathway recognition and stimulate cytotoxic CD8⁺ T cell alloimmunity could be transferred into naive recipients. When MHC class II-deficient C57BL/6 DCs were cocultured with BALB/c DCs, they were unable to promote the CD8⁺ T cell alloimmune cytotoxicity seen with wild-type C57BL/6 DCs. Thus, the authors demonstrated that for the generation of CD8⁺ T cell alloimmune responses, recipient APCs can present intact MHC alloantigen and copresent processed allopeptide, linking the direct and indirect pathways and recruiting CD4⁺ T cell help.