CD28 co-stimulation drives memory phenotype (MP) Treg cell and MP CD4+Foxp3− effector T cell homeostatic proliferation.

Abstract

Abstract Foxp3+Tregs, CD4+Foxp3− effector T cells and CD8+ T cells are composed of naïve (NP, CD44loCD62Lhi) and memory (MP, CD44hiCD62Llo) subsets. About 30% of MP T cells are cycling (Ki-67+) in-vivo. To determine the factors that drive T cell homeostatic proliferation in-vivo, we investigated the roles of cytokines (TNFa, IL-2, IL-7, and IL-33), TCR-MHC-II interactions, co-stimulatory (CD28, ICOS, CD40, GITR, OX40), and co-inhibitory (CTLA-4, PD1, BTLA, TIGIT) receptors. Blockade of CD28-CD80/86 signaling completely inhibited MP Treg and MP CD4+Foxp3− effector T cell proliferation but did not affect MP CD8+ T cell proliferation. Inhibition of cytokine activity did not affect the proliferation of any of the T cell populations. Marked enhancement of homeostatic proliferation of MP Treg and MP CD4+Foxp3−T cells was observed by blocking TCR-MHCII interactions, CTLA4-CD80/CD86 interactions, or by agonistic mAbs to CD40, GITR, and OX40. Blocking PD1-PDL1 interactions modestly enhanced proliferation, while blocking BTLA, TIM3, ICOS and TIGIT did not affect MP T cell proliferation. Importantly, blocking of CD28-CD80/86 signaling completely abrogated the augmentation of proliferation observed after blocking TCR and CTLA4, PD1, or by agonistic mAbs to GITR, OX40, and CD40. Our results support a complex pathway in which CD28-CD80/CD86 interactions play a critical role in Treg and MP CD4+Foxp3−T cells proliferation in-vivo. CTLA4-CD80/CD86 interactions act as a brake, while MHCII-TCR interactions negatively regulate the inhibitory functions of CTLA-4. These findings have important implications for the use of biologic agents to enhance Treg numbers/functions, as well as for the use of checkpoint inhibitors. Supported by DIR, NIAID.