Impact of Staphylococcus aureus intramammary infection on cellular proliferation and apoptosis on developing mammary glands of pregnant dairy heifers

Mammary epithelial cell (MEC) proliferation and stromal regression are critical processes that govern mammary gland development during first gestation in dairy heifers, directly influencing future lactational performance. Intramammary infections (IMI) can alter tissue architecture in nonlactating mammary glands, but their impact on mammary cellular dynamics during gestation remains unclear. This study quantified MEC and stromal cell proliferation and apoptosis in infected and healthy mammary glands at distinct late-gestation stages (6.5-, 7.5-, and 8.5 mo pregnant). Twenty-one pregnant Holstein heifers were utilized. One culture negative quarter of each heifer was infused with saline (SAL), while the contralateral quarter was infused with Staphylococcus aureus (CHALL). Heifers were euthanized 21 d post-challenge, and mammary tissue samples were collected from center parenchyma, located just above the gland cistern, and edge parenchyma, located near the mammary fat pad. Proliferating cells were identified using immunohistochemical detection of Ki-67, and apoptotic cells were identified using the TUNEL assay. Proliferation of MEC was elevated in the center region CHALL quarters at 6.5 mo of gestation, suggesting early activation of tissue repair in response to infection; however, this response was not sustained at later stages. In contrast, MEC proliferation was reduced in CHALL quarters compared with SAL in edge tissues at 6.5 mo, which may indicate impaired epithelial expansion in the edge region during early infection. Increased stromal cell proliferation was observed in the central parenchyma of CHALL quarters at 8.5 mo, indicating delayed stromal involution and potential impairment of alveolar development. Apoptosis of stromal cells increased as gestation progressed and was modestly elevated in CHALL quarters, possibly reflecting immune cell turnover due to persistent inflammation. Apoptosis in epithelial cells was unaffected by IMI, in contrast to findings in hormonally stimulated, nonpregnant heifers. These results suggest that IMI during gestation can alter mammary remodeling and may compromise secretory activation near parturition, with potential implications for future lactational performance.