Bystander Activation and Anti-Tumor Effects of CD8+ T Cells Following Interleukin-2 Based Immunotherapy Is Independent of CD4+ T Cell Help

We have previously demonstrated that immunotherapy combining agonistic anti-CD40 and IL-2 (IT) results in synergistic anti-tumor effects. IT induces expansion of highly cytolytic, antigen-independent "bystander-activated" (CD8(+)CD44high) T cells displaying a CD25(-)NKG2D(+) phenotype in a cytokine dependent manner, which were responsible for the anti-tumor effects. While much attention has focused on CD4(+) T cell help for antigen-specific CD8(+) T cell expansion, little is known regarding the role of CD4(+) T cells in antigen-nonspecific bystander-memory CD8(+) T cell expansion. Utilizing CD4 deficient mouse models, we observed a significant expansion of bystander-memory T cells following IT which was similar to the non-CD4 depleted mice. Expanded bystander-memory CD8(+) T cells upregulated PD-1 in the absence of CD4(+) T cells which has been published as a hallmark of exhaustion and dysfunction in helpless CD8(+) T cells. Interestingly, compared to CD8(+) T cells from CD4 replete hosts, these bystander expanded cells displayed comparable (or enhanced) cytokine production, lytic ability, and in vivo anti-tumor effects suggesting no functional impairment or exhaustion and were enriched in an effector phenotype. There was no acceleration of the post-IT contraction phase of the bystander memory CD8(+) response in CD4-depleted mice. The response was independent of IL-21 signaling. These results suggest that, in contrast to antigen-specific CD8(+) T cell expansion, CD4(+) T cell help is not necessary for expansion and activation of antigen-nonspecific bystander-memory CD8(+) T cells following IT, but may play a role in regulating conversion of these cells from a central memory to effector phenotype. Additionally, the expression of PD-1 in this model appears to be a marker of effector function and not exhaustion.