Osteopontin: a novel cytokine involved in the regulation of Mycobacterium avium subspecies paratuberculosis infection in periparturient dairy cattle.

Osteopontin (Opn), an important mediator of the cell-mediated immune response, enhances the host immune response against mycobacterial infections. Infections caused by Mycobacterium avium ssp. paratuberculosis (MAP) have a devastating effect on the dairy industry. We sought to characterize Opn at the level of gene and protein expression in periparturient dairy cows naturally infected with MAP. Peripheral blood mononuclear cells (PBMC) were isolated from control, subclinical, and clinical periparturient dairy cows naturally infected with MAP beginning 3 wk precalving to 5 wk postcalving and incubated with medium alone (non-stimulated: NS), concanavalin A (ConA), or a whole-cell sonicate of MAP (MPS). Real-time PCR was performed to evaluate expression of Opn and classical Th1 and Th2 cytokines. Results demonstrated greater Opn expression in nonstimulated PBMC isolated from subclinical cows compared with control and clinical cows. For clinical cows, there was a strong correlation between Opn expression and expression of the Th1 cytokines IFN-gamma and IL-1 alpha for nonstimulated PBMC and IFN-gamma and IL-12 for PBMC stimulated with MPS. Expression of tumor necrosis factor-alpha was greater in clinical cows than the other groups. Nonstimulated, ConA, and MPS-stimulated PBMC from subclinical cows secreted more IFN-gamma, and MPS-stimulated PBMC from clinical cows secreted more IL-4 compared with the other groups. Immunoblot analysis of PBMC detected 4 Opn proteins at 60, 52, 34, and 27 kDa. This is the first study to evaluate the role of Opn on the immune response of dairy cows naturally infected with MAP, and results suggest Opn may be a key regulator against MAP infection.