The expression of interleukin-2 (IL-2) has been implicated in the modulation of the outcome of ocular infection with herpes simplex virus type 1 (HSV-1); however, its effects remain controversial. To clarify the role of IL-2, we constructed a recombinant HSV-1 (HSV-IL-2) that expresses two copies of the murine IL-2 gene under the control of the latency-associated transcript (LAT) promoter of HSV-1 in a LAT-negative virus. In tissue culture, the replication of the HSV-IL-2 was 100-fold lower than that of the wild-type virus at a low multiplicity of infection (MOI). Addition of recombinant anti-IL-2 polyclonal antibody markedly enhanced HSV-IL-2 replication in tissue culture. In the 7-day period after ocular infection of BALB/c mice, the replication of HSV-IL-2 was significantly lower than that of wild-type virus in tear cultures, whole eyes, and brain, but was equivalent to wild-type replication in the trigeminal ganglia. Ocular challenge of BALB/c mice with HSV-IL-2 alone, at an MOI that resulted in only 13% survival when parental virus was used, was associated with 90% survival. This decrease in virulence was further shown to be attributable to the expression of IL-2 by coinfection of mice with HSV-IL-2 and the parental virus. This resulted in a decrease in virulence of the parental virus (5% survival when administered alone versus 50% survival on coinfection with HSV-IL-2). The survival of HSV-IL-2-infected mice was compromised by depletion of either IL-2, CD4(+), or CD8(+) T cells (50% survival) and abolished completely by depletion of both T-cell subtypes. Moreover, depletion of CD4(+) T cells, CD8(+) T cells, or both increased the titers of HSV-IL-2 in the tears, eyes, trigeminal ganglia, and brains of infected mice, so that titers were equivalent to or higher than that of the parental virus. These results suggest that IL-2 expression by recombinant HSV-1 reduces virulence and that depletion of IL-2 or T cells increases virulence in HSV-1-infected mice.