Investigating ruminal resident immune cells in dairy cattle before and after a subacute ruminal acidosis challenge

Immune cells have been observed in many tissues of the mammal including the gut, liver, adipose, bone, and neural tissue. Our objective was to demonstrate whether immune cells are present within the stratified rumen epithelium of healthy lactating dairy cattle. We hypothesized that immune cells would be found in the rumen tissue and that the total amount of leukocytes present would increase during a ruminal acidosis challenge. We conducted a 2-phase experiment with 9 rumen-cannulated lactating Holstein cows. During phase 1, cows were fed a diet containing 31% NDF, 24% forage NDF, and 27% starch (CON) for 14 d. The starch sources were corn silage and dry ground corn. During phase 2, cows were fed a diet with 28% NDF, 16% forage NDF, and 32% starch (acid induction diet, AID) for 7 d. The AID starch sources were corn silage, dry ground corn, ground wheat, and ground barley. During both phases, rumen fluid, fecal, blood, and rumen tissue samples were collected. Feed intake and milk yield were recorded each day and milk samples were collected at each milking during the final 2 d of each period. Data were analyzed with mixed models that included the fixed effect of phase (CON or AID) and the random effect of cow. The AID treatment reduced rumen pH and increased area under the curve for rumen pH beneath 5.8 and 5.6. Feed intake, milk yield, and milk protein yield increased for AID. Phase did not affect ruminal immune cell phenotype proportions or total presence. Most immune cells observed were CD3⁺ (T cells) for both CON and AID. The ruminal immune cells were located within the lamina propria, stratum basale, and stratum spinosum of the rumen papillae. Our data demonstrate that T lymphocytes are prominent in rumen tissue. Understanding the role of ruminal leukocytes may yield new insights into ruminant gut health, function, development, and maintenance.