Fever is an evolutionarily conserved response to infection. But exactly how it confers survival benefits has been unclear. A new study reported in Immunity describes a molecular mechanism to explain how fever promotes T cell trafficking and enhances immune surveillance during infection, through a thermal sensory pathway involving heat shock protein 90 (HSP90) and α4 integrins.

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Exposing mouse T cells to febrile temperatures (40 oC) for 12 hours led to significant increases in their ability to adhere to ligands for α4β1 integrin (VCAM1) and α4β7 integrin (MAdCAM1) but not to β2 integrin ligand (ICAM1). Chemokine-induced transmigration across VCAM1-coated or MAdCAM1-coated membranes was also markedly increased following pretreatment of T cells at 40 oC compared with 37 oC. This suggested that thermal stress specifically promotes α4 integrin-mediated T cell adhesion and transmigration.

As expected, T cells treated at temperatures >38.5 oC also showed a marked upregulation of various HSPs. However, immunoprecipitation studies revealed that only HSP90 selectively bound to α4 integrin — an interaction that was enhanced by exposure to febrile temperatures. Consistent with a functional role for HSP90–α4 integrin binding, HSP90 overexpression promoted α4 integrin-dependent T cell adhesion and transmigration at 37 oC.

Mutational studies showed that HSP90 binds to the α4 integrin cytoplasmic tail, with one mutation (R985A) specifically abolishing the interaction. Accordingly, T cells from mice expressing the R985A α4 integrin mutant (Itga4R985A/R985A mice) failed to show thermal stress-induced adhesion and transmigration in vitro. And following transfer into mice, wild-type T cells, but not Itga4R985A/R985A T cells, pretreated at 40 oC showed increased ‘sticking’ to high endothelial venules and homing to inguinal lymph nodes. Increased T cell trafficking to draining lymph nodes was also observed in wild-type mice after exposure to whole body hyperthermia for 6 hours compared with Itga4R985A/R985A mice treated the same way.

Delving deeper into the mechanism, the authors first used a fluorescence resonance energy transfer (FRET) system to show that HSP90 binding induces the active (extended) conformation of α4 integrins. Moreover, thermal stress or HSP90 overexpression promoted binding of talin and kindlin 3, which are the two critical co-activators of inside-out α4 integrin activation. Indeed, silencing of talin and kindlin 3 inhibited thermal stress-induced activation of α4 integrins. Second, they used a bimolecular fluorescence complementation technique to show that fever-induced binding of HSP90 induces dimerization and clustering of α4 integrins on the cell membrane. Dimerization of the two α4 integrin subunits was mediated by simultaneous binding by the amino-terminal domain and the carboxy-terminal domain of HSP90. Integrin clustering supported activation of a signalling pathway involving focal adhesion kinase (FAK) and Ras homologue gene family, member A (RHOA) GTPase to promote cell migration.

Finally, the authors tested whether the HSP90–α4 integrin axis is protective in the setting of oral Salmonella enterica subsp. enterica serovar Typhimurium infection. The infection was more severe, with more intestinal tissue damage and bacterial dissemination, in Itga4R985A/R985A mice than in wild-type mice. And increased severity was associated with a reduction in the number of T cells, and monocytes, accessing the inflamed tissue and some associated lymphoid tissues.

HSP90 binding induces the active (extended) conformation of α4 integrins

So, the fever-induced HSP90–α4 integrin axis is crucial for promoting immune cell trafficking to inflamed tissues to facilitate the clearance of bacterial infection. Whether this pathway can be targeted therapeutically — to enhance or temper immune trafficking — awaits further study.