• Mark W Julian, Heather R Strange, Megan N Ballinger, Richard S Hotchkiss, Tracey L Papenfuss, and Elliott D Crouser.
• Dorothy M. Davis Heart and Lung Research Institute, Division of Pulmonary Allergy, Critical Care, and Sleep Medicine, Wexner Medical Center, Columbus, OH, United States of America.
• Plos One. 2015 Jan 1; 10 (7): e0132921.

ObjectiveImmune suppression during critical illness predisposes to serious infections. We sought to determine the mechanisms regulating tolerance and cross-tolerance to common pro-inflammatory danger signals in a model that recapitulates the intact in vivo immune response.Materials And MethodsFlt3-expanded splenocytes obtained from wild-type or matching IRAK-M knockout (IRAK-M-/-), C57BL/6, male mice (8-10 weeks old) were treated repeatedly or alternately with either LPS or CpGA DNA, agonists of Toll-like receptor (TLR)-4 and -9, respectively, over successive 24-hour periods. Supernatants were collected following each 24-hour period with cytokine release (ELISA) and splenocyte IRAK-M expression (Western blot) determined. Tolerance and cross-tolerance were assessed in the absence or presence of programmed death receptor (PD)-1 blocking antibody or IL-7 pre-treatment.Main ResultsSplenocytes notably exhibited both tolerance and cross-tolerance to subsequent treatments with either LPS or CpGA DNA. The character of tolerance and cross-tolerance in this model was distinct following initial LPS or CpGA treatment in that TNFα and IFNγ release (not IL-10) were suppressed following LPS; whereas, initial CpGA treatment suppressed TNFα, IFNγ and IL-10 release in response to subsequent stimulation (LPS or CpGA). Tolerance and cross-tolerance were unrelated to IL-10 release or PD-1 but were attenuated in IRAK-M-/- splenocytes. IL-7 significantly suppressed IRAK-M expression and restored TNFα and IFNγ production without influencing IL-10 release.ConclusionsIn summary, acute immune tolerance and cross-tolerance in response to LPS or CpGA were distinct in that LPS selectively suppressed pro-inflammatory cytokine responses; whereas, CpGA suppressed both pro- and anti-inflammatory responses. The induction of tolerance and cross-tolerance in response to common danger signals was mechanistically unrelated to IL-10 or PD-1 but was directly influenced by IRAK-M expression. IL-7 reduced IRAK-M expression and attenuated immune tolerance induced by either LPS or CpGA, and thus may be useful for reversal of immune tolerance in the setting of critical illness.

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