HISTOLOGIC RESPONSE OF THE HEIFER MAMMARY-GLAND TO INTRAMAMMARY INFUSION OF INTERLEUKIN-2 OR INTERFERON-GAMMA

The histologic and cytologic responses of heifer mammary glands infected with Staphylococcus aureus were studied after infusion of interleukin-2 or interferon-gamma. Two groups of 4 heifers each, which were infected experimentally with S. aureus in all 4 mammary quarters, were infused in diagonal quarters with 7.5 x 10(5) units of interleukin-2 or 10(5) units of interferon-gamma; remaining quarters received PBS. Heifers in both trials were slaughtered 14 d after cytokine infusion, and mammary tissues were collected for histological examination. Uninfected quarters from 2 additional heifers were left untreated to compare infected with uninfected tissues for both trials. Morphologic evaluation and leukocyte infiltration scores were performed on tissue sections stained with hematoxylin and eosin, and plasma cells were quantified on sections stained with immunoperoxidase. Infected quarters had lower percentages of alveolar epithelial and luminal areas and higher percentages of stromal area than did uninfected quarters in the interleukin-2 trial, but no differences were observed between infected quarters that had been treated with PBS or interleukin-2. Likewise, interferon-gamma treatment had no effect on mammary parenchymal components in the infected quarters. Interleukin-2 treatment significantly elevated leukocytosis into the mammary gland parenchyma compared with infected quarters treated with PBS and uninfected quarters. Among the leukocyte types evaluated, eosinophilic infiltration was elevated in interleukin-2 quarters over that of PBS controls. In both trials, concentrations of plasma cells bearing Ig were elevated in infected versus uninfected quarters. Plasma cell concentrations also were higher in cytokine than PBS controls, especially in interleukin-2 quarters. Results suggested that neither cytokine influenced the histology of infected mammary tissues, but both interleukin-2 and interferon-gamma increased, although insignificantly, the prevalence of all isotypes of plasma cells bearing Ig, suggesting enhancement of the local immune response to IMI.