Bystander memory CD8 T cell proliferation after anti-CD40/IL-2 treatment is independent of CD4 T cells

Systemic cancer immunotherapy combining agonistic antiCD40 and interleukin 2 results in synergistic anti-tumor effects with marked antigen independent expansion of bystander memory CD8 T cells displaying anti-tumor abilities. Our lab has previously shown that this expansion coincides with a loss of peripheral CD4 T cells due to activation induced cell death. While much research to date has focused on the effects of CD4 T cells on antigenspecific CD8 T cell expansion, little is known regarding the role of CD4 T cells in bystander CD8 T cell expansion. Utilizing models of CD4 knockout mice as well as CD4 depleting antibodies we observed a significant expansion of memory CD8 T cells displaying a CD25-NKG2D+ bystander phenotype following immunotherapy, similar to non-depleted mice. Interestingly, the expanded bystander memory population was enriched from cells of the effector memory phenotype and up regulated Tim-3 and PD-1 in the absence of CD4 T cells. However, they also displayed comparable cytokine production and lytic ability suggesting no functional impairment or exhaustion. While Tim-3 and PD-1 expression have previously been linked to exhaustion, the phenotype described here is consistent with their other known role as acute activation markers on effector/effector memory T cells. These results suggest that CD4 T cells may not be necessary for the expansion and activation of antigen-nonspecific bystander memory CD8 T cells under conditions of strong immune stimulation yet may play a role in regulating the conversion of these bystander cells from a central to effector memory phenotype in secondary lymphoid organs. Authors’ details Dermatology, University of Califonia, Davis, Sacramento, CA, USA. Radiation Oncology, University of Califonia, Davis, Sacramento, CA, USA. Pediatrics, University of Minnesota, Minneapolis, MN, USA.