Inducing Immunity with Direct Cell Reprogramming for Cancer Immunotherapy

Immunotherapy has revolutionized cancer treatment, but the majority of patients do not benefit from current strategies. Antigen presentation and lymphoid-mediated cytotoxicity are essential immune properties to elicit antitumor immune responses and control tumor growth. However, high tumor heterogeneity and immune evasion mechanisms impair tumor immune surveillance. High frequencies of cytotoxic natural killer (NK) cells and antigen-presenting conventional dendritic cell type 1 (cDC1) in the tumor environment correlate with positive responses to immunotherapy and survival. However, current therapeutic strategies to enhance innate lymphoid cytotoxicity and antigen presentation function continue to fall short of their potential.

Direct cell reprogramming mediated by transcription factors (TFs) offers the possibility to induce immune functional properties in other cell types, allowing fast and efficient generation of effector cells for therapy. Ectopic expression of the TFs PU.1, IRF8 and BATF3 (PIB) induces cDC1 fate in fibroblasts, but instructor TFs for lymphoid programs have not been reported.

Here, I employed direct reprogramming strategies to impose antigen presentation function in cancer cells and to identify TF codes for innate cytotoxic function.Enforced expression of PIB imposed cDC1 morphology and immunophenotype in mouse and human tumor cells generating tumor antigen presenting cells (tumor-APCs) with reduced tumorigenicity. Tumor-APCs showed extensive transcriptional remodeling with upregulation of antigen-presenting machinery genes. cDC1 reprogramming activated surface expression of antigen presentation complexes and co-stimulatory molecules, enabling presentation of endogenous tumor antigens. Functionally, tumor-APCs acquired competence to engulf, process and cross-present antigens to CD8+ T cells. Adoptive transfer of tumor-APCs to melanoma tumors controlled tumor growth and increase survival, an effect that was synergistic with immune checkpoint blockade. To determine instructor factors for innate cytotoxic function, I overexpressed four canonical NK cell TFs- TBET, ETS1, NFIL3, EOMES (TENE) in human embryonic fibroblasts. TENE-transduced cells activated the expression of the critical NK marker CD56 and acquired morphological changes such as the defining intracellular granules. Furthermore, TENE induced the production and secretion of cytotoxic molecules, granzyme B and granulysin, and pro-inflammatory cytokines TNF-α and IL-2. Induced cytotoxic lymphocytes exhibit global transcriptional changes towards an NK cell program, with upregulation of immunomodulatory genes. Finaly, I showed that TENE-reprogramming was conserved across human dermal and mouse embryonic fibroblasts.

Overall, the findings described here lay the foundation for the development of immunotherapies centered on the induction of immune cell properties in non-immune cell types. In the future, conversion of tumor cells into cDC1 can be utilized in vivo to attenuate tumorigenesis and elicit antitumor immune responses in situ. Instructor TFs for innate cytotoxicity may be harnessed for reinvigorate exhausted NK cells or to confer killing capacity to tumor resident cells.