Exhausted Immune Cells Spread Dysfunction Like Infection — and a Key Enzyme Stops It

Monocytes are frontline immune sentinels that normally shift from activation to resolution after an infection clears. But under conditions of persistent inflammation — such as sepsis or chronic infection — they enter a state called monocyte exhaustion: high in inflammatory markers like CD38 and PD-L1, low in immune-activating markers like CD86, and broadly dysfunctional. Until now, how this exhausted state propagates through the immune system has been poorly understood.

Using mouse bone marrow-derived monocytes and human PBMCs, researchers at Virginia Tech induced exhaustion with prolonged high-dose LPS stimulation (100 ng/mL for 5 days) and then co-cultured these exhausted cells with naïve monocytes. Within just 5 hours, naïve monocytes exposed to exhausted cells showed significant upregulation of exhaustion markers — CD38, CD157, and PD-L1 — and a corresponding reduction in the immune-activating marker CD86. Transwell experiments confirmed this transmission required direct cell-to-cell contact rather than diffusible factors. Connexin 43 (Cx43) gap junctions were identified as the physical conduit: monocytes from Cx43 knockout mice failed to propagate exhaustion, and pharmacological Cx43 inhibition blocked the transfer. In adoptive transfer experiments in live mice, exhausted donor monocytes similarly induced exhaustion markers in recipient naïve monocytes in vivo, corroborating the in vitro findings.

Exhausted monocytes also severely affected vascular endothelial cells. Co-culture with exhausted monocytes increased endothelial apoptosis, elevated expression of the adhesion molecules ICAM-1 and VCAM-1, and enhanced monocyte transmigration across endothelial monolayers — hallmarks of endothelial dysfunction that underlie atherosclerosis and vascular inflammation. Blocking CD38 with the small-molecule inhibitor 78c markedly reduced these effects, as did Cx43 inhibition, suggesting that exhaustion-to-endothelium crosstalk is also contact-dependent and CD38-mediated. Separately, exhausted monocytes suppressed T cell proliferation and activation in co-culture, an immunosuppressive effect also reversed by CD38 inhibition.

Mechanistically, the study maps a sustained positive feedback loop: LPS triggers TLR4 signaling through the TRAM-TRIF adaptor complex, activating Src kinase and mTORC1 (via Raptor recruitment and S6K activation). mTORC1 then drives STAT1/STAT3 signaling, which transcriptionally induces CD38. Elevated CD38 depletes intracellular NAD⁺ — a critical metabolic cofactor — which impairs mTORC2 activity, reducing Akt phosphorylation and suppressing downstream PGC1α/β and CREB, molecules associated with immune competence and mitochondrial health. Treatment with rapamycin (mTOR inhibitor) partially restored monocyte function by downregulating CD38, PD-L1, and STAT1/STAT3/S6K signaling, while boosting CD86 expression. Notably, rapamycin showed stronger rescue effects on mTORC1-driven exhaustion markers than on mTORC2-associated pathways, consistent with its known selectivity.

These findings are significant for understanding immune collapse in sepsis, cancer, and chronic inflammatory disease. The CD38–mTOR axis emerges as a druggable hub that not only drives exhaustion within individual monocytes but orchestrates its propagation through the broader immune and vascular environment. The study is limited by its primary reliance on in vitro systems and a single mouse model, and further work is needed to validate these mechanisms in human disease settings.