Transforming growth factor-beta induced by live or ultraviolet-inactivated equid herpes virus type-1 mediates immunosuppression in the horse

Up to 21 days after exposure to live or ultraviolet-inactivated equid herpesvirus type-1 (EHV-1) autologous serum from ponies caused an immunosuppressive effect if incorporated into T-cell proliferation assays to EHV-1. The suppressive factor in the sera of ponies also inhibited T-cell response to phytohaemagglutinin. Increased levels of circulating activated transforming growth factor-beta, (TGF-beta(1)) were detected, and the suppressive activity of the serum could be reversed by antibody to TGF-beta(1). In a challenge experiment the ponies which exhibited circulating TGF-beta(1) activity succumbed to infection while the ones with similar magnitudes of T-cell responses, but no TGF-beta(1) activity, were protected. A definition of this immunosuppressive mechanism and its mode of induction must be central to the design of vaccines and to an understanding of the pathogenesis of EHV-1.