Anatomic compartmentalization of a cytokine buffer system promotes anti-viral immune defense

Introduction/Rationale: The circulating concentration of the proinflammatory cytokine IL-6 is regulated by a blood buffer system formed by molar excess of the soluble forms of the IL-6 receptor (sIL-6R) and co-receptor gp130 (sgp130). These buffer components serve to rapidly capture free IL-6 released following immune challenge, preventing systemic proinflammatory signaling. How then can IL-6 accumulate at mucosal sites as needed for immune defense?

Methods: Post-sublethal H1N1 infection, lungs and sera were collected on Day 0 and Day7 from B6 (control mice) and transgenic sgp130fc mice that overexpress sgp130 in the lung fluid. IL-6 and buffer agents (sIL-6R and sgp130) were measured in the sera and the lung homogenate by ELISA. scRNA-seq was performed on non-infected and infected lung cells to define intrinsic capacity of lung cells to generate buffer agents. High resolution confocal imaging at millimeter special scale was used to measure IL-6 induced signaling in the presence and absence of forced buffer expression in the lung.  Influenza-based RT-PCR was used to measure viral load in the lung after sublethal H1N1 infection and 1xLD50 dose was used to evaluate survivorship with and without IL-6 buffer activity in the lung tissue.

Results: We find that the IL-6 buffer system is anatomically compartmentalized, and is substantially reduced within lung interstitial fluid, enabling IL-6 to accumulate in the lung mucosa within a mouse model of influenza virus infection. Genetic transplantation of IL-6 capture activity into lung tissue limits spatial diffusion of this cytokine and lowers immune protection against influenza virus.

Conclusion: Our results point to anatomic compartmentalization of biological buffer systems as a homeostatic mechanism, in this case, enabling mucosal immune defense without dangerous systemic delivery of the proinflammatory signal.