Histological changes of conjunctiva-associated lymphoid tissue caused by pneumonia in Holstein young cows.

Conjunctiva-associated lymphoid tissue (CALT) induces mucosal IgA production on the ocular surface and within the respiratory mucosa. Increased CALTs have been reported in cows. The conjunctiva represents a potential site for mucosal vaccinations against respiratory infections in animals due to the strong immunological relationship between the conjunctiva and the respiratory mucosae. However, it remains unclear whether respiratory antigen sensitization triggers CALT development and a subsequent conjunctival humoral immune response, particularly in cows. To clarify these mechanisms, we compared CALT morphology between pneumonia-infected cows with chronic respiratory antigen stimulation and healthy cows. There were no differences in the appearance rate, area, or immune cell distribution of CALT between the groups. The percentage of discontinuous epithelial layers covering the third eyelid lymphoid follicle was significantly higher in the cows with pneumonia than in the healthy cows. The numbers of IgA-positive cells in the eyelid and third eyelid conjunctival lamina propria were significantly higher in cows with pneumonia. Similarly, these animals exhibited numerous pulmonary IgA-positive cells. These results suggest that chronic respiratory antigen sensitization during pneumonia promotes antigen uptake and mucosal immune effector function in the bovine conjunctiva through immune interactions between the conjunctiva and respiratory mucosa.