Multispecific responses by T cells expanded by endogenous self-peptide/MHC complexes

The paradox of autoreactivity to self-peptides in physiological as opposed to pathological immune responses is not well understood. Here, we directly examined the human T cell response to endogenous self-peptides in a series of healthy subjects. CFSE-labeled T cells were stimulated with unmanipulated antigen-presenting cells containing endogenous self-antigen, and the resulting CD4+ populations entering into cell cycle (CFSE(low)) or non-proliferating CD4+ cells (CFSE(high)) were single-cell sorted, cloned and screened against a panel of self-antigens and microbial recall antigens to interrogate their antigen reactivity. The percentage of CD4+ T cells entering cell cycle in response to self-peptide/MHC was calculated to be 0.04%, and entry into cell cycle was dependent upon CD28 costimulation. Clones derived from CFSE(low) T cells exhibited significantly greater cross-reactivity to multiple antigens than CFSE(high) clones or other CD4+ clones generated after microbial antigen stimulation. Sequencing the TCRbeta chains indicated that CFSE(low) clones were indeed clonal. These data demonstrate that T cell clones generated on stimulation by endogenous self-peptides exhibit a high degree of multispecificity, and we speculate that their multispecificity is based upon recognition of shared-backbone MHC determinants.