cDC1 prime and are licensed by CD4+ T cells to induce anti-tumour immunity

CD4 T cells are thought to help promote anti-tumour responses by ‘licensing’ antigen presenting cells (APCs) that activate CD8 T cells. Conventional type 1 dendritic cells (cDC1s) are responsible for cross-presentation of tumour-derived antigens to CD8 T cells. Prevailing models presume that the cDC1 is licensed by CD4 T cells that are themselves activated by a distinct cDC subset, the cDC2. The recent finding that neoantigens presented by major histocompatibility complex (MHC) class II molecules can promote rejection of tumours that lack MHC class II (MHC-II) surface expression is consistent with an indirect action of CD4 T cells, such as cDC1 licensing. However, no study has directly identified the APC that primes the CD4 T cells responsible for licensing or clearly identified the target of CD4 licensing in vivo. Here, we generated cDC1-specific Cre expressing mouse strain to inactivate or induce expression of MHC-I, MHC-II, or CD40 specifically within the cDC1 lineage. Using a tumour model that relies on CD8 T cells and CD4 T cells for rejection, we discovered that early priming of CD4 T cells against tumour-derived antigens, in contrast to soluble antigens, relied overwhelmingly on the cDC1 and not the cDC2. cDC1 do not simply transport antigen to lymph nodes for processing by cDC2, since lack of MHC-II expression on cDC1 prevented CD4 T cell priming. We also found that CD40 signaling not only affects licensing of cDC1 for CD8 T cell priming, but is also critical for the activation of CD4 T cells. Thus, in the setting of tumour-derived antigens, cDC1 can function as an autonomous platform, capable of priming both CD4 and CD8 T cells and orchestrating their cross-talk required for optimal anti-tumour immunity. We used microarrays to obtain and compare gene expression signatures for homeostatic cDC1 populations in the presence or absence of CD40 expression. cDC1 from spleen or skin draining lymph nodes of Xcr1+/+ or Xcr1Cre/+ CD40fl/fl mice were sort-purified for RNA extraction and hybridization on Affymetrix microarrays.