Using X-ray Crystallography, Biophysics, and Functional Assays to Determine the Mechanisms Governing T-cell Receptor Recognition of Cancer Antigens

Molecular characterization of HLA class II binding to the LAG-3 T cell co-inhibitory receptor

Immune checkpoint inhibitors (antibodies that block the T cell co-inhibitory receptors PD-1/PD-L1 or CTLA-4) have revolutionized the treatment of some forms of cancer. Importantly, combination approaches using drugs that target both pathways have been shown to boost the efficacy of such treatments. Subsequently, several other T cell inhibitory receptors have been identified for the development of novel immune checkpoint inhibitors. Included in this list is the co-inhibitory receptor lymphocyte activation gene-3 (LAG-3), which is upregulated on T cells extracted from tumor sites that have suppressive or exhausted phenotypes. However, the molecular rules that govern the function of LAG-3 are still not understood. Using surface plasmon resonance combined with a novel bead-based assay (AlphaScreenTM ), we demonstrate that LAG-3 can directly and specifically interact with intact human leukocyte antigen class II (HLA-II) heterodimers. Unlike the homologue CD4, which has an immeasurably weak affinity using these biophysical approaches, LAG-3 binds with low micromolar affinity. We further validated the interaction at the cell surface by staining LAG-3+ cells with pHLA-II-multimers. These data provide new insights into the mechanism by which LAG-3 initiates T cell inhibition.

T cell receptor alpha variable 12-2 bias in the immunodominant response to Yellow fever virus

The repertoire of human αβ T-cell receptors (TCRs) is generated via somatic recombination of germline gene segments. Despite this enormous variation, certain epitopes can be immunodominant, associated with high frequencies of antigen-specific T cells and/or exhibit bias toward a TCR gene segment. Here, we studied the TCR repertoire of the HLA-A*0201-restricted epitope LLWNGPMAV (hereafter, A2/LLW) from Yellow Fever virus, which generates an immunodominant CD8+ T cell response to the highly effective YF-17D vaccine. We discover that these A2/LLW-specific CD8+ T cells are highly biased for the TCR α chain TRAV12-2. This bias is already present in A2/LLW-specific naïve T cells before vaccination with YF-17D. Using CD8+ T cell clones, we show that TRAV12-2 does not confer a functional advantage on a per cell basis. Molecular modeling indicated that the germline-encoded complementarity determining region (CDR) 1α loop of TRAV12-2 critically contributes to A2/LLW binding, in contrast to the conventional dominant dependence on somatically rearranged CDR3 loops. This germline component of antigen recognition may explain the unusually high precursor frequency, prevalence and immunodominance of T-cell responses specific for the A2/LLW epitope.

Thermal Stability of Heterotrimeric pMHC Proteins as Determined by Circular Dichroism Spectroscopy

T cell receptor (TCR) recognition of foreign peptide fragments, presented by peptide major histocompatibility complex (pMHC), governs T-cell mediated protection against pathogens and cancer. Many factors govern T-cell sensitivity, including the affinity of the TCR-pMHC interaction and the stability of pMHC on the surface of antigen presenting cells. These factors are particularly relevant for the peptide vaccination field, in which more stable pMHC interactions could enable more effective protection against disease. Here, we discuss a method for the determination of pMHC stability that we have used to investigate HIV immune escape, T-cell sensitivity to cancer antigens and mechanisms leading to autoimmunity.