Approximately 10% of peripheral T cells express 2 functional TCR αβ heterodimers. Receptor co-expression changes the repertoire of TCRs produced during thymic development, enabling generation of T cells bearing TCRs not capable of mediating positive selection or that would normally be negatively selected. The effect of receptor co-expression on the composition and functionality of the peripheral TCR repertoire is not well defined, though evidence demonstrates dual TCR cells pose an increased risk for unwanted immune responses such as autoimmunity and alloreactivity. Based on our previous finding that dual TCR expression promotes positive selection, we hypothesized that dual TCR expression may enhance T cell homeostasis via increased reactivity against self-peptide:MHC (pMHC) ligands. To examine the effect of dual TCR expression on T cell homeostasis, we performed cotransfer experiments comparing T cells genetically deficient for dual TCR expression (TCRα^+/- ) with wild-type T cells in models of acute and chronic lymphopenia-induced proliferation (LIP). Lack of dual TCR expression resulted in reduced LIP. The effect of dual TCR expression on LIP was most pronounced in acute lymphopenia, which is driven by recognition of low-affinity self-pMHC ligands. Differences in homeostatic proliferation were not attributable to differences in total TCR expression or signaling, but were dependent on interaction with MHC and associated with increased affinity for positively selecting self-pMHC as evidenced by higher expression of CD5 by dual TCR cells from wild-type mice. These results represent an unappreciated novel mechanism driving homeostasis and shaping the T cell repertoire, potentially promoting autoreactive or heterologous immune responses.

Defective post-transplantation thymopoiesis is associated with chronic graft-versus-host disease (GVHD), a multiorgan pathology affecting up to 80% of patients after allogeneic hematopoietic stem cell transplantation (HSCT). Previous work demonstrated that the subset of T cells expressing 2 T cell receptors (TCRs) is predisposed to alloreactivity, driving selective and disproportionate activity in acute GVHD in both mouse models and HSCT patients. Here we investigate a potential role for this pathogenic T cell subset in chronic GVHD (cGVHD). HSCT patients with cGVHD demonstrated increased numbers of dual TCR cells in circulation. These dual receptor cells had an activated phenotype, indicating an active role in cGVHD. Notably, single-cell RNA sequencing identified the increased dual TCR cells in cGVHD as predominantly expressing Tbet, indicative of a proinflammatory phenotype. These results identify dual TCR cells as specific mediators of pathogenic inflammation underlying cGVHD and highlight Tbet-driven T cell function as a potential pathway for potential therapeutic targeting.