CD40 agonism enhances immune checkpoint blockade and generates immunologic memory via CD4 T cells in ERα+ mammary tumors.

There has been marked improvement in the clinical outcome of triple-negative breast cancer (TNBC) with the use of immune checkpoint blockade (ICB) although serious immune-related adverse effects are not uncommon. Unlike TNBC, ERα + breast tumors are largely unresponsive to ICB. Here we demonstrate defective priming by cross-presenting conventional dendritic cells (cDCs) and a blunted response to ICB in ERα + mouse mammary tumors compared to TNBC. Systemic administration of an agonistic CD40 antibody (aCD40) induced T cell proliferation and activation in tumor-draining lymph nodes and attracted effector T cells to the tumor bed from the periphery. This effect was largely due to activation, maturation and migration of type 1 conventional dendritic cells (cDC1s). aCD40 alone slowed tumor growth in ERα + tumors but its combination with ICB cured tumor-bearing mice, accomplishing a "vaccine effect" and the immune-mediated rejection of tumor rechallenge. The anti-tumor effect of aCD40 effect was cDC1 and CD8 + T cell-dependent, whereas the rejection of secondary tumor rechallenge in cured mice required CD4 + T cells. Importantly, intra-tumoral administration of aCD40 combined with systemic or intra-tumoral ICB - to mimic neoadjuvant therapeutic approaches-induced complete regressions of both treated and distant tumors. These findings indicate that aCD40 achieves DC activation required for the response to immunotherapy in ERα + tumors and further supports intra-tumoral administration of both aCD40 and ICB as an effective treatment that might limit systemic exposure and lower risk of immune-related toxicity.