Objective: Fever is a cardinal feature of the systemic response to infection or trauma; however, the molecular mechanisms underlying the proposed protective action of fever remain largely unknown. Previously, we have shown that fever-range thermal stress causes increased trafficking of lymphocyte across high endothelial venules (HEV) in secondary lymphoid organs via an IL-6-dependent mechanism that increases the intravascular display of the gatekeeper homing molecule, ICAM-1. The present study assessed the role of IL-6 downstream signaling pathways (ERK1-2, STAT3/1) in augmenting ICAM-1 expression on HEV during thermal stress.

Method: These experiments utilized gp130^757F and gp130^ΔSTAT mice which express mutated forms of gp130, the common signal-transducing receptor subunit for IL-6 cytokines. Gp130^757F mice are deficient in SHP2 and SOCS3 binding to gp130 and subsequent recruitment of the Ras-Raf-MEK-ERK pathway, due to a tyrosine to phenylalanine (Y→F) substitution at residue 757. In gp130^ΔSTAT mice, mutation (Y→F substitution) at residue 765 and deletion (Δ769→917) prevents STAT3 and STAT1 binding and activation at four requisite tyrosine docking sites in the gp130 cytoplasmic domain.

Results: PNAd⁺ cuboidal HEVs were identified in lymph nodes of both gp130^757F and gp130^ΔSTAT mice, indicating that the gp130 mutations do not interfere with the development of these vascular structures. HEV from wild-type and gp130^757F mice were fully competent for ICAM-1 induction through a gp130-independent pathway using TNF. Strong ICAM-1 induction was also observed in gp130^757F mice following treatment with fever-range whole body hyperthermia, suggesting that the MEK-ERK pathway is dispensable for ICAM-1 upregulation in HEV by IL-6. In contrast, fever-range temperature failed to augment intravascular ICAM-1 expression in gp130^ΔSTAT mice, indicating a non-redundant role for STAT3/1 activation for thermal upregulation of ICAM-1 on HEV. Phosphorylation of STAT3 was further observed in the lymphoid organs of wild-type mice following fever-range thermal stress or administration of IL-6. These findings are consistent with the proposed model of thermal regulation of ICAM-1 in HEV by IL-6 through a STAT3-dependent induction of icam-1 gene transcription in high endothelial cells.

Conclusion: These studies provide insight into the signal transduction pathways underlying enhancement of lymphocyte trafficking during systemic thermal stress. (Supported by NIH grants CA79765, CA094045).