P. A. Sieling

It has long been the paradigm that T cells recognize peptide antigens presented by major histocompatibility complex (MHC) molecules. However, nonpeptide antigens can be presented to T cells by human CD1b molecules, which are not encoded by the MHC. A major class of microbial antigens associated with pathogenicity are lipoglycans. It is shown here that human CD1b presents the defined mycobacterial lipoglycan lipoarabinomannan (LAM) to alpha beta T cell receptor-bearing lymphocytes. Presentation of these lipoglycan antigens required internalization and endosomal acidification. The T cell recognition required mannosides with alpha(1-->2) linkages and a phosphotidylinositol unit. T cells activated by LAM produced interferon gamma and were cytolytic. Thus, an important class of microbial molecules, the lipoglycans, is a part of the universe of foreign antigens recognized by human T cells.

<h3></h3> Toll-like receptors (TLRs) are pattern recognition molecules expressed by cells of the innate immune system that mediate inflammatory responses by recognizing microbial patterns, including lipopolysaccharide and bacterial flagellin. TLRs are thought to influence adaptive immunity indirectly by enhancing antigen presentation; however, pattern recognition molecules are also expressed on cells of the adaptive immune system. Therefore, to investigate the ability of TLR ligands to directly induce adaptive immune responses, we evaluated the expression and function of TLRs on human T cells. We found that human peripheral blood T cells and T cell clones express TLRs and exposure of T cells to TLR ligands enhanced their response to T cell receptor activation. Bacterial flagellin, a microbial pattern commonly used for motility by enteric pathogens and a TLR5 ligand, was a potent inducer of primary human T cells and T cell clones. To investigate the mechanism whereby T cells respond to TLR ligation, we used an immortalized human T cell line, Jurkat. We found that flagellin activates a signaling pathway in T cells involving p38 mitogen-activated protein (MAP) kinase, and that in combination with TCR activation, p38 MAP kinase is activated to a greater extent. The potentiating effect of bacterial flagellin for TCR-activation also resulted in the production of IL-2 from Jurkat cells, indicating that TLR ligands act as a costimulatory signal for T cells. Our data suggest that microbial pattern molecules may directly activate the adaptive immune response by acting as an adjuvant for antigen-specific T cells. The presence of bacterial flagellin on bacteria in the intestine may potentiate inflammation by directly activating T cells.

SUMMARY Langerhans cells express the nonpolymorphic antigen-presenting molecule CD1a, positioning them as contributors to host immunity against Mycobacterium leprae in human leprosy. CD1a was originally shown to present non-canonical lipopeptide antigens such as dideoxymycobactin and chemically diverse hydrophobic ligands. Here, we generated CD4⁺ T cell lines from leprosy lesions that recognized M. leprae in a CD1a-restricted manner. Unexpectedly, antigen recognition was protease-sensitive, prompting biochemical purification that identified two microbial protein antigens: LppX, a 25-kDa lipoglycoprotein, and Ag85A, a 30-kDa secreted protein with no known lipid modification. Recombinant proteins activated the corresponding T cell lines in a CD1a-dependent manner. Epitope mapping identified 12-mer peptides that fully reconstituted antigenicity, were conserved between M. leprae and M. tuberculosis , and elicited robust, dose-dependent IFN-γ production and T cell proliferation, establishing that DNA-encoded, ribosomally translated peptides serve as CD1a-restricted cognate antigens. Biochemical analyses showed peptide binding to CD1a, supported by isoelectric focusing and surface plasmon resonance ( K D ∼75 μM for Ag85A). CD1a–peptide tetramers specifically stained cognate T cells, soluble CD1a was sufficient to present peptide antigen, and transfer of the LppX-specific TCR into naïve T cells restored antigen responsiveness. Using CD1a–peptide tetramers, we identified antigen-specific T cells enriched in patients undergoing reversal reactions compared with patients with lepromatous leprosy and healthy donors. The CD1a-restricted T cell lines secreted IFN-γ and IL-26, cytokines with established antimicrobial activity. Together, these findings demonstrate that CD1a can present canonical microbial peptides as part of a cell-mediated immune response in leprosy, extending the known spectrum of CD1a ligands. Because CD1a is nonpolymorphic and presents antigens to antimicrobial T cells, CD1a–peptide complexes may provide a broadly applicable platform for studying, detecting, and potentially targeting mycobacterial immunity.