Model of the effects of CsA treatment on HTLV-1 infection. (A) During the natural course of infection, infected lymphocytes spread early during infection before a immune response is formed. After a full immune response is formed, the spread of infected lymphocytes is controlled. (B) CsA treatment before HTLV-1 infection decreases the HTLV-1–specific cell-mediated immune response, allowing for infected lymphocytes to produce more virus. The increase in virus production early leads to an increase in the proviral load in early infection. (C) CsA treatment after infection significantly reduces virus expression, in turn leading to a decrease in the number of HTLV-1–infected lymphocytes over the course of infection.