The lower intestine of adult mammals is densely colonized with non-pathogenic (commensal) microbes. Gut bacteria induce protective immune responses, which ensure host-microbial mutualism. The continuous presence of commensal intestinal bacteria has made it difficult to study mucosal immune dynamics. Here we report a reversible germ-free colonization system in mice that is independent of diet or antibiotic manipulation. A slow (>14 days) onset of a long-lived (t1/2>16 weeks), highly specific anti-commensal IgA response in germ-free mice was observed. Ongoing commensal exposure in colonized mice rapidly abrogated this response. Sequential doses lacked a classical prime-boost effect seen in systemic vaccination, but specific IgA induction occurred as a stepwise response to current bacterial exposure, such that the antibody repertoire matched the existing commensal content.

During immune challenge, T lymphocytes engage pathways of anabolic metabolism to support clonal expansion and the development of effector functions. Here we report a critical role for the non-essential amino acid serine in effector T cell responses. Upon activation, T cells upregulate enzymes of the serine, glycine, one-carbon (SGOC) metabolic network, and rapidly increase processing of serine into one-carbon metabolism. We show that extracellular serine is required for optimal T cell expansion even in glucose concentrations sufficient to support T cell activation, bioenergetics, and effector function. Restricting dietary serine impairs pathogen-driven expansion of T cells in vivo, without affecting overall immune cell homeostasis. Mechanistically, serine supplies glycine and one-carbon units for de novo nucleotide biosynthesis in proliferating T cells, and one-carbon units from formate can rescue T cells from serine deprivation. Our data implicate serine as a key immunometabolite that directly modulates adaptive immunity by controlling T cell proliferative capacity.

Abstract:A prerequisite for establishment of mutualism between the host and the dense microbial

How systemic metabolic alterations during acute infections impact immune cell function remains poorly understood. We found that acetate accumulates in the serum within hours of systemic bacterial infections and that these increased acetate concentrations are required for optimal memory CD8(+) T cell function in vitro and in vivo. Mechanistically, upon uptake by memory CD8(+) T cells, stress levels of acetate expanded the cellular acetyl-coenzyme A pool via ATP citrate lyase and promoted acetylation of the enzyme GAPDH. This context-dependent post-translational modification enhanced GAPDH activity, catalyzing glycolysis and thus boosting rapid memory CD8(+) T cell responses. Accordingly, in a murine Listeria monocytogenes model, transfer of acetate-augmented memory CD8(+) T cells exerted superior immune control compared to control cells. Our results demonstrate that increased systemic acetate concentrations are functionally integrated by CD8(+) T cells and translate into increased glycolytic and functional capacity. The immune system thus directly relates systemic metabolism with immune alertness.