Effect of in utero heat stress on neonatal small intestinal cell turnover, and markers associated with barrier function and immunoglobulin G transport in newborn dairy heifers.

Late gestation heat stress in dairy cows reduces offspring's capacity to absorb colostrum immunoglobulin G (IgG). This study evaluated small intestine histomorphology, cellular turnover, and gene expression profiles of in utero heat stressed versus in utero cooled heifers at birth. For the last 54 ± 5 d of gestation, pregnant dams were housed in a free stall barn and were either heat stressed (shade of the barn) or cooled (shade, fans, and water soakers) during a subtropical summer. Heat stressed pregnant cows responded physiologically by elevating respiration rate and skin temperature relative to their cooled counterparts (+25 breaths per minute and +1.7°C) and gave birth to heifers who were in utero heat stressed (IUHS) or in utero cooled (IUCL), respectively (n = 8/group). Heifers were euthanized at birth (before colostrum feeding) to harvest gastrointestinal tract (GIT) tissues. Histomorphology (villi length, width, and crypt depth) was assessed from duodenum, jejunum, and ileum tissue portions of the GIT, via H&E staining. Cellular turnover (death/proliferation) was quantified in crypt and villi regions of the jejunum and ileum via immunohistochemistry. The fluorescence intensity of tight junction proteins (occludin and zona occludens-1) and the neonatal Fc receptor were assessed in the jejunum and ileum. Jejunum and ileum tissue samples were snap-frozen for gene expression analysis of endosomal (FCGRT, DAB2, MAMDC4), tight junction (TJP1, TJP2, TJP3, OCLN, CLDN1), heat shock (HSP90AA1, HSF1), and autophagy (ATG3, ATG5) related genes by qPCR. Data were analyzed using PROC MIXED or PROC GLIMMIX. There were no differences in villi length or width across intestinal sections, nor crypt depth in the duodenum and jejunum; however, ileal crypts were shorter in IUHS heifers. No differences between treatments were detected in the rate of proliferation within either tissue section. Apoptosis was increased crypts of the jejunum and villi of the ileum in IUHS heifers, but unchanged in ileal crypts and jejunal villi. Occludin abundance tended to be reduced in the jejunum of IUHS heifers only, whereas other tight junction proteins were largely unchanged. Abundance of the neonatal Fc receptor was decreased in the jejunum of IUHS heifers and not different between treatments in the ileum. Gene expression analysis indicated a tendency for upregulation of CLDN1 and significant downregulation of TJP2 in the jejunum, and a significant upregulation of MAMDC4, ATG5, and HSF1 in the ileum of IUHS heifers. Additionally, expression of tight junction genes OCLN, TJP1, TJP2, and TJP3 were upregulated in the ileum of IUHS heifers. Overall, IUHS had minimal effects on histomorphology but disrupted cellular turnover and tight junction regulation, with reduced neonatal Fc receptor abundance in the jejunum, providing insight to mechanisms that may be responsible for impaired colostrum IgG absorption. These findings suggest that impaired passive immunity in IUHS calves may be attributed to disruptions in intestinal cellular and molecular function rather than gross structural changes.