In a model of parasite-mediated exhaustion, stem-like CD8 T cells differentiate into an unconventional intermediate effector memory subset.

CD8 T cell exhaustion has been reported in mice susceptible to Toxoplasma gondii infection. While the differentiation of CD8 exhausted subsets has been extensively reported, most of these studies have been conducted in chronic viral and cancer models. During chronic T. gondii infection, phenotypic and transcriptomic analyses of the polyclonal antigen-specific CD8 T cell response characterize 4 populations based on KLRG1 and CD62L expression. Pop1 (KLRG1+CD62Llo) bears the attributes of a terminal effector subset, and pop2 (KLRG1-CD62Llo) is similar to effector memory CD8 T cells. Akin to chronic viral infection and cancer systems, pop3 (KLRG1-CD62Lhi) exhibits the characteristics of stem-like progenitor CD8 T cells (high Tcf7, Slamf6, and Cxcr5 expression), whereas pop4 (KLRG1+CD62Lhi) closely resembles a transitory subset (elevated Tbx21, low Tcf1, and Tox expression). During chronic viral infection, the stem-like progenitor CD8 T cells transition into a terminally differentiated exhausted subset via an intermediate population. However, in our system, pop3 (KLRG1-CD62Lhi) generates pop4 (KLRG1+CD62Lhi), which does not convert into a conventional terminally differentiated exhausted subset but instead transitions into effector pop1 (KLRG1+CD62Llo). Notably, during the chronic phase of the infection, pop1 cannot retain its functionality, irrespective of its origin, which may hamper its ability to control reactivation. Our observations emphasize that the differentiation of exhausted CD8 T cells in non-viral infections, like chronic toxoplasmosis, follows a different pattern than established models and highlights the need to develop new immune strategies better tailored for a broad range of pathogens.