“…Thus, it was initially surprising that T cells with a "memory phenotype" increase with age in unimmunized inbred laboratory animals kept in specific pathogen-free (SPF) conditions (Surh and Sprent, 2008;Chiu et al, 2013;Nikolich-Žugich, 2018). Moreover, CD8 and CD4 T cells of memory phenotype accumulate in germfree (GF) mice not exposed to foreign or commensal microbes, and even in GF mice on elemental diets that lack food antigens (Pereira et al, 1986;Dobber et al, 1992;Haluszczak et al, 2009;White et al, 2017;Kawabe et al, 2020). The "memory phenotype" CD8 T cells express CD44, the "signature" memory cell marker, as well as elevated adhesion molecules (CD18, CD49d), activation-associated molecules (CD69), and survival cytokine receptors (CD122, CD132).…”