PHORBOL ESTER STIMULATION OF EQUINE MACROPHAGE CULTURES ALTERS EXPRESSION OF EQUINE INFECTIOUS-ANEMIA VIRUS

Equine infectious anemia virus (EIAV) is a lentivirus that replicates predominantly in mature tissue macrophages. Viral expression is strong ly influenced by the state of differentiation of the host cell. While blood monocytes can be infected, viral transcription is limited until the cell differentiates into a mature macrophage. Activation of mature macrophages infected with EIAV might also alter viral expression, pre sumably through binding of cellular transcription factors to viral nuc leic acid sequences within the long terminal repeat (LTR). Using DNA a mplification techniques, we compared LTR sequences of U.S. field strai ns of EIAV to sequences of a laboratory adapted strain of the virus. A ll field strain sequences were more closely related to Wyoming strain than to the Malmquist laboratory adapted strain or a previously sequen ced infectious molecular clone of EIAV. Primary equine monocyte-derive d macrophage cultures were infected with virulent and avirulent strain s of EIAV and the effects of macrophage stimulation on EIAV expression were determined. Stimulation of macrophages with phorbol ester activa ted the cells to secrete tumor necrosis factor alpha (TNF alpha). This activation signal also resulted in a significant downregulation of vi ral expression as determined by supernatant reverse transcriptase acti vity. This effect occurred independent of the virulence of the virus s train used or the nucleic acid sequence of the viral LTR. This may rep resent an adaptive response of EIAV to evade the host immune response and establish a persistent infection.