Deciphering the ontogeny of CD4+ resident memory T cells that globally seed the skin and protect against cutaneous leishmaniasis

Dermal resident memory CD4+ T cells (dTrm) are uniquely positioned to protect against vector-borne pathogens such as Leishmania major. However, despite their significance, the ontogeny of dTrm cells and the influence of the skin microbiome and T cell intrinsic signals that promote their retention following entry, remain unclear and often controversial. We aim to define how dTrm cells develop and use the information to optimize dTrm cell-mediated immunity in vaccines. In our preliminary studies, we identified protective Th1-biased CD4+ dTrm cells in mice immunized with an experimental peptide vaccine based on an immunodominant leishmanial antigen, phosphoenolpyruvate carboxykinase (PEPCK). Curiously, while a PEPCK-mRNA- lipid nanoparticle (LNP) vaccine promoted the entry of PEPCK-specific T cells into the skin, the T cells were not maintained. We will leverage this novel finding to discover the factors required for developing and maintaining protective dTrm cells globally seeded in the skin. We will: 1) Determine if precursor dTrm cells develop before entry into the skin. This information will provide information on how to expand this population in a vaccine; 2) Identify the role of the skin microbiome in the development and retention of dTrm cells in the skin. The skin microbiome has a large influence on T cells in the skin, and we will test how bacteria influence both the development and retention of dTrm cells; and 3) Expand the pool of protective dTrm cells in the skin. Based on our findings, we will test several approaches to expand the dTrm cell population. These include altering the cytokine milieu and inducing the expression of essential transcription factors for dTrm cell development. These studies will be done in murine models, taking advantage of cutting-edge technology to interrogate T cell responses in immunized mice, including identifying critical genes and proteins associated with dTrm development by cellular indexing of transcriptomes and epitopes, flow cytometry, deletion of genes by CRISPR/Cas9 editing, ectopic expression of genes by retroviral transduction, and in vivo targeted mRNA-LNP delivery of critical factors to promote Trm cells. Leishmaniasis is a neglected disease that causes a wide range of clinical entities. Drugs to cure leishmaniasis are often ineffective and toxic, and there is no vaccine for the disease. The inability to create a protective vaccine is partly because protection is mediated by T cells, rather than antibodies– the target for most vaccines. We have shown that dTrm cells can provide long-lasting protection in mice that have healed a primary infection, and we hypothesize that vaccines that expand a dTrm cell population will lead to an effective vaccine. Thus, these studies have direct relevance to developing a vaccine for leishmaniasis. Additionally, there are many vector-borne diseases with no vaccines, and we believe that expanding dTrm cells in vaccines for other vector-borne diseases will be essential for optimal protection against a wide range of infections.