A simple assay to quantify mycobacterial lipid antigen-specific T cell receptors in human tissues and blood.

T cell receptors (TCRs) encode the history of antigenic challenge within an individual and have the potential to serve as molecular markers of infection. In addition to peptide antigens bound to highly polymorphic MHC molecules, T cells have also evolved to recognize bacterial lipids when bound to non-polymorphic CD1 molecules. One such subset, germline-encoded, mycolyl lipid-reactive (GEM) T cells, recognizes mycobacterial cell wall lipids and expresses a conserved TCR-ɑ chain that is shared among genetically unrelated individuals. We developed a quantitative PCR assay to determine expression of the GEM TCR-ɑ nucleotide sequence in human tissues and blood. This assay was validated on plasmids and T cell lines. We tested blood samples from South African subjects with or without tuberculin reactivity or with active tuberculosis disease. We were able to detect GEM TCR-ɑ above the limit of detection in 92% of donors but found no difference in GEM TCR-ɑ expression among the three groups after normalizing for total TCR-ɑ expression. In a cohort of leprosy patients from Nepal, we successfully detected GEM TCR-ɑ in 100% of skin biopsies with histologically confirmed tuberculoid and lepromatous leprosy. Thus, GEM T cells constitute part of the T cell repertoire in the skin. However, GEM TCR-ɑ expression was not different between leprosy patients and control subjects after normalization. Further, these results reveal the feasibility of developing a simple, field deployable molecular diagnostic based on mycobacterial lipid antigen-specific TCR sequences that are readily detectable in human tissues and blood independent of genetic background.

CD1 proteins: targets of T cell recognition in innate and adaptive immunity.

The CD1 family consists of antigen presenting molecules encoded by genes located outside of the major histocompatibility complex. CD1 proteins are conserved among mammalian species and are expressed on the surface of cells involved in antigen presentation. The CD1 system has been shown to be involved in activation of cell-mediated responses, and T cells specific for either CD1 molecules or antigens presented by CD1 have been isolated. Structural and biochemical analyses demonstrate that antigens presented by CD1 are nonpeptide lipid or glycolipid structures, including examples found in the cell walls of pathogenic mycobacteria. The hydrophobic part of these antigens most likely binds in the CD1 ligand-binding groove, whereas the polar headgroup of these antigens appears to make direct contact with the T cell receptor and determines specific recognition. Presentation of antigens by CD1 molecules requires uptake and intracellular processing by antigen presenting cells and can be achieved for both exogenous and endogenous antigens. T cells recognizing CD1 restricted antigens have a broad range of functional activities that suggest that the CD1 system is involved in both innate and adaptive immune responses against microbial infections.

The mannose receptor delivers lipoglycan antigens to endosomes for presentation to T cells by CD1b molecules.

We have characterized the CD1b-mediated presentation pathway for the mycobacterial lipoglycan lipoarabinomannan (LAM) in monocyte-derived antigen-presenting cells. The macrophage mannose receptor (MR) was responsible for uptake of LAM. Antagonism of MR function inhibited both the internalization of LAM and the presentation of this antigen to LAM-reactive T cells. Intracellular MRs were most abundant in early endosomes, but they also were located in the compartment for MHC class II antigen loading (MIIC). Internalized LAM was transported to late endosomes, lysosomes, and MIICs. MRs colocalized with CD1b molecules, suggesting that the MR could deliver LAM to late endosomes for loading onto CD1b. LAM and CD1b colocalized in organelles that may be sites of lipoglycan antigen loading. This pathway links recognition of microbial antigens by a receptor of the innate immune system to the induction of adaptive T cell responses.