Multi-Stage Regulation of CD8+ T Lymphocyte Differentiation by TGF-Beta

Abstract

Epithelial resident memory CD8+ T cells (eTRM) have emerged as critical immune sentinels, playing a dominant role in protecting against barrier-invading pathogens and tumors, yet also mediating pathogenesis of autoimmune disorders. While their functional importance is established, determinants of eTRM differentiation have yet to be fully elucidated. One critical cytokine in eTRM differentiation is TGF-b, which is known to have a wide range of consequences on CD8+ T cell fate. A major regulator of this pleiotropism is the context by which TGF-b is activated from its latent form- thought to be tightly regulated by alphaV-integrins expressed on many cell types, including epithelial and dendritic cells (DC). Therefore, we sought to investigate the control of TGF-b activation during eTRM differentiation. We initially hypothesized that DC in the skin activate TGF-b for eTRM formation. Upon conditional deletion of TGF-b-activating alphaV-integrin in DC, we observed a profound deficiency in skin eTRM that was not due to effects in the skin during terminal differentiation or priming. Instead, we found that at homeostasis, DC-activated TGF-b on naïve CD8+ T cells epigenetically conditions them for eTRM formation in lymph nodes through non-cognate interactions of migratory DC with naïve CD8+ T cells. Collectively, our studies identify a novel stage of T cell differentiation, during which the pre-immune repertoire is actively conditioned for a specialized fate. Although terminal eTRM differentiation was independent of DC-activated TGF-b, other sources of TGF-b in the skin may regulate factors involved in long-term tissue retention. We therefore studied spatiotemporal dynamics to understand contributions of local TGF-b in the skin to eTRM differentiation. Through intravital microscopy of the skin, we found that TGF-b controls T cell migration into the epidermis via hair follicles by induction of chemokine receptor CCR6. This uncovers a new role for TGF-b in regulating spatial organization of cells within tissues. Finally, to extend our studies of spatiotemporal dynamics, we developed a GFP-Smad2 reporter system to further probe TGF-b signaling nuances in T cells in vivo. Together, these studies provide novel insights into the cellular dynamics mediating eTRM development and bolster our understanding of TGF-b signaling in the adaptive immune system.