In-vitro effect of heat stress on bovine monocytes lifespan and polarization.

Heat stress (HS) has a negative impact on dairy cows' health, milk production, reproductive performance and immune defenses. Cellular and molecular responses to high temperatures in bovine polymorphonuclear cells and peripheral blood mononuclear cells (PBMCs) have been investigated so far. On the contrary, the effects of high temperatures on isolated monocytes remain almost undisclosed. The aim of this study was to unravel the in vitro effects of high temperatures, simulating a severe HS related body hyperthermia, on bovine lifespan and M1/M2 polarisation. The PBMCs were isolated from whole blood of 9 healthy dairy cattle. Monocytes were sorted by magnetic activated cell sorting and cultured over night at 39 °C (normothermia) or 41 °C (HS). Apoptotic rate and viability were assessed and mRNA abundance for heat shock proteins (HSPs), heat transcription factors (HSFs) and genes involved in monocyte/macrophage polarization (STAT1, STAT2, STAT3, STAT6, IL1β, TGF1β, IL-10, COX2) were quantified by qPCR. We found that apoptosis increased in monocytes exposed to 41 °C, as compared to control, while viability conversely decreased. HS increased the abundance of HSF1 and HSP70. The concomitant decrease of STAT1 and STAT2 and the increase of STAT6 genes abundance at 41 °C suggest, at transcriptional factors level, a polarization of monocytes from a classical activated M1 to a non-classically activated M2 monocytes. In conclusion, the exposure of bovine monocytes to high temperatures affects their lifespan as well as the abundance of genes involved in HS response and in monocyte/macrophages polarization phenotype, confirming that bovine immune response may be significantly affected by hyperthermia.