Studies on human B cells have featured CD27 as a marker and mediator of the B cell response. We have studied CD27 expression and function on B cells in the mouse. We find that B cells acquire CD27 at the centroblast stage and lose it progressively upon further differentiation. It is not a marker for somatically mutated B cells and is present at very low frequency on memory B cells. Enrichment of CD27 among centroblasts and the presence of its ligand CD70 on occasional T and B cells in or near germinal centers (GCs) suggested a role for CD27/CD70 interactions in clonal B cell expansion. Accordingly, GC formation in response to influenza virus infection was delayed in CD27 knockout mice. CD27 deficiency did not affect somatic hypermutation or serum levels of virus-specific IgM, IgG, and IgA attained in primary and recall responses. Adoptive transfer of T and B cells into CD27/CD28^−/− mice revealed that CD27 promotes GC formation and consequent IgG production by two distinct mechanisms. Stimulation of CD27 on B cells by CD28⁺ Th cells accelerates GC formation, most likely by promoting centroblast expansion. In addition, CD27 on T cells can partially substitute for CD28 in supporting GC formation.

Despite their low abundance in the tumor microenvironment (TME), classical type 1 dendritic cells (cDC1) play a pivotal role in anti-cancer immunity, and their abundance positively correlates with patient survival. However, their interaction with CD4⁺ T-cells to potentially enable the cytotoxic T lymphocyte (CTL) response has not been elucidated. Here we show that contact with activated CD4⁺ T-cells enables human ex vivo cDC1, but no other DC types, to induce a CTL response to cell-associated tumor antigens. Single cell transcriptomics reveals that CD4⁺ T-cell help uniquely optimizes cDC1 in many functions that support antigen cross-presentation and T-cell priming, while these changes don’t apply to other DC types. We robustly identify “helped” cDC1 in the TME of a multitude of human cancer types by the overlap in their transcriptomic signature with that of recently defined, tumor-infiltrating DC states that prove to be positively prognostic. As predicted from the functional effects of CD4⁺ T-cell help, the transcriptomic signature of “helped” cDC1 correlates with tumor infiltration by CTLs and Thelper(h)−1 cells, overall survival and response to PD-1-targeting immunotherapy. These findings reveal a critical role for CD4⁺ T-cell help in enabling cDC1 function in the TME and may establish the helped cDC1 transcriptomic signature as diagnostic marker in cancer.

CD4⁺ T cells can "help” or "license” conventional type 1 dendritic cells (cDC1s) to induce CD8⁺ cytotoxic T lymphocyte (CTL) anticancer responses, as proven in mouse models. We recently identified cDC1s with a transcriptomic imprint of CD4⁺ T-cell help, specifically in T-cell-infiltrated human cancers, and these cells were associated with a good prognosis and response to PD-1-targeting immunotherapy. Here, we delineate the mechanism of cDC1 licensing by CD4⁺ T cells in humans. Activated CD4⁺ T cells produce IFNβ via the STING pathway, which promotes MHC-I antigen (cross-)presentation by cDC1s and thereby improves their ability to induce CTL anticancer responses. In cooperation with CD40 ligand (L), IFNβ also optimizes the costimulatory and other functions of cDC1s required for CTL response induction. IFN-I-producing CD4⁺ T cells are present in diverse T-cell-infiltrated cancers and likely deliver “help” signals to CTLs locally, according to their transcriptomic profile and colocalization with “helped/licensed” cDCs and tumor-reactive CD8⁺ T cells. In agreement with this scenario, the presence of IFN-I-producing CD4⁺ T cells in the TME is associated with overall survival and the response to PD-1 checkpoint blockade in cancer patients.

Allogeneic stem cell transplantation (alloSCT), following induction chemotherapy, can be curative for hemato-oncology patients due to powerful graft-versus-tumor immunity. However, disease recurrence remains the major cause of treatment failure, emphasizing the need for potent adjuvant immunotherapy. In this regard, dendritic cell (DC) vaccination is highly attractive, as DCs are the key orchestrators of innate and adaptive immunity. Natural DC subsets are postulated to be more powerful compared with monocyte-derived DCs, due to their unique functional properties and cross-talk capacity. Yet, obtaining sufficient numbers of natural DCs, particularly type 1 conventional DCs (cDC1s), is challenging due to low frequencies in human blood. We developed a clinically applicable culture protocol using donor-derived G-CSF mobilized CD34⁺ hematopoietic progenitor cells (HPCs) for simultaneous generation of high numbers of cDC1s, cDC2s and plasmacytoid DCs (pDCs). Transcriptomic analyses demonstrated that these ex vivo-generated DCs highly resemble their in vivo blood counterparts. In more detail, we demonstrated that the CD141⁺CLEG9A⁺ cDC1 subset exhibited key features of in vivo cDC1s, reflected by high expression of co-stimulatory molecules and release of IL-12p70 and TNF-α. Furthermore, cDC1s efficiently primed alloreactive T cells, potently cross-presented long-peptides and boosted expansion of minor histocompatibility antigen-experienced T cells. Moreover, they strongly enhanced NK cell activation, degranulation and anti-leukemic reactivity. Together, we developed a robust culture protocol to generate highly functional blood DC subsets for in vivo application as tailored adjuvant immunotherapy to boost innate and adaptive anti-tumor immunity in alloSCT patients.