We studied the innate and adaptive immune system of rhesus macaques infected with the virulent simian immunodeficiency virus isolate SIVmac251 by evaluating natural killer (NK) cell activity, cytokine levels in plasma, humoral and virological parameters, and changes in the activation markers CD25 (interleukin 2R The immune system of higher vertebrates consists of innate and adaptive components. Innate immunity exhibits immediate recognition and response without prior sensitization. Cells of the innate immune system (i.e., monocytes/macrophages, natural killer [NK] cells, and polymorphonuclear leukocytes) recognize pathogen-associated molecular patterns and activate events such as phagocytosis, induction of the synthesis of antimicrobial peptides, expression of inflammatory and effector cytokines and chemokines, induction of nitric oxide synthase in macrophages, and expression of costimulatory molecules on antigen-presenting cells. The adaptive immune system uses somatically generated antigen receptors that are clonally distributed on T and B lymphocytes. Generally, adaptive immune recognition in the absence of innate immune recognition results in inactivation of lymphocytes that express receptors involved in the identification events (20). Thus, innate immune responses have critical consequences in adaptive immune responses.[Little is known of the contribution of the innate immune system during infection with the human immunodeficiency virus (HIV). Based on similarities of biologic and genetic features, simian immunodeficiency virus (SIV) infection of rhesus macaques provides the best animal model of HIV infection and AIDS. Accordingly, this animal model is critical for the elucidation of mechanisms of pathogenesis and for the development of vaccines and antiviral therapies (12). As with almost all viral infections, the innate immune system is thought to be the first component of the immune system that recognizes SIV infection. However, few studies have methodically analyzed the changes induced in cell phenotype and cytokine levels by SIV infection. Recent studies have demonstrated that SIV infection results in a generalized increase in lymphocyte turnover (23) and that the primary site for viral replication is activated memory CD4 ϩ T cells that are present in the intestinal lamina propia (46). Although cellular changes are not that dramatic at this early stage in peripheral lymphoid tissue, peripheral blood (PB) and lymph nodes (LN) still reflect the pathologic changes induced by the viral infection and are readily available for longitudinal studies.To analyze changes in the activation state of cells from the innate and adaptive immune system after SIV infection, we evaluated NK activity, cytokine levels in plasma, and changes in activation markers on lymphoid cells of rhesus macaques after infection with pathogenic SIVmac251. We found the sequential appearance in plasma of interferon-␣/␤ (IFN-␣/␤) interleukin-18 (IL-18) and IL-12, whereas IL-4, IFN-␥ and granulocyte-macrophage colony-stimulating factor (GM-CS...

Interleukin 18 (IL-18) is a proinflammatory cytokine expressed by several cell types, including activated dendritic cells and macrophages, that acts in synergy with IL-12 as an important amplifying factor for IFN-gamma production and Th1 development. To study the immunological and virological effects of IL-18 expression in the context of a lentiviral infection, we inoculated rhesus macaques with a high dose of replication-competent simian immunodeficiency virus (SIV) vectors carrying the rhesus IL-18 gene in the sense (SIV(IL-18)) or antisense (SIV(FIGI)) orientation. Both vectors behaved as attenuated viruses, resulting in low viral loads, induction of low and transient levels of inflammatory cytokines, no CD4(+) T cell depletion, and mild activation of T lymphocytes. Although IL-18-expressing virus could be isolated from some SIV(IL18)-infected macaques for 12 weeks postinfection, the anti-SIV humoral and cellular immune responses of macaques inoculated with SIV(IL18) and SIV(FIGI) were similar to each other, with the exception of an early IFN-gamma response in animals infected with SIV(IL18). In summary, expression of IL-18 during the acute phase of SIV infection does not increase viral replication or influence the outcome of the antiviral immune response.

Human immunodeficiency virus infection is characterized by dysregulation of antigen-presenting cell function and defects in cell-mediated immunity. Recent evidence suggests that impaired ability of CD4؉ T cells to upregulate the costimulatory molecule CD154 is at the core of this dysregulation. To test the hypothesis that increased expression of CD154 on infected CD4؉ T cells could modulate immune function, we constructed a replication-competent simian immunodeficiency virus (SIV) vector that expressed CD154. We found that this recombinant vector directed the expression of CD154 on the surface of infected CD4؉ T cells and that expression of CD154 resulted in activation of B cells present in the same cultures. Experimental infection of rhesus macaques resulted in very low viral loads for the CD154-expressing virus and the control virus, indicating that expression of CD154 did not result in increased viral replication. Analyses of the anti-SIV immune responses and the phenotype of lymphocytes in blood and lymphoid tissues showed changes that occurred during the acute phase of infection only in animals infected with the CD154-expressing SIV, but that became indistinguishable from those seen in animals infected with the control virus at later time points. We conclude that the level of expression of CD154 in itself is not responsible for affecting the immune response to an attenuated virus. Considering that the CD154-expressing SIV vector and the virus control did not carry an active nef gene, our results suggest that, in CD4 ؉ T cells infected with wild-type virus, Nef is the viral factor that interferes with the immune mechanisms that regulate expression of CD154.

Interleukin-18 (IL-18), previously known as interferon-gamma (IFN-gamma)-inducing factor (IGIF), is a proinflammatory cytokine expressed by activated macrophages that acts in synergy with IL-12 as an important amplifying factor for IFN-gamma production and Th1 development. To study the effect of IL-18 on a lentiviral infection, we cloned the IL-18 gene from a rhesus macaque and constructed replication-competent simian immunodeficiency virus (SIV) that expressed either the precursor pro-IL-18 (SIV(IL-18)) or the mature form (SIV(mIL-18)) of IL-18. The predicted amino acid sequence for rhesus IL-18 had 96% homology with the human one, differing in only 8 of 193 residues. SIV(IL-18) and SIV(mIL-18) replicated more slowly than control viruses in the CEM x 174 cell line and resulted in the development of chronically infected cell lines that expressed high levels of infectious SIV. The cell line generated by SIV(IL-18) released large quantities of IL-18 into the supernatant, whereas the one obtained from SIV(mIL-18) showed the accumulation of IL-18 in the cytoplasm. Similarly, SIV(IL-18) and SIV(mIL-18) replicated more slowly than the unmodified viral vector in rhesus peripheral blood mononuclear cells (PMBC), but only SIV(IL-18) expressed biologically active IL-18. These experiments show that the precursor form of IL-18 is necessary for the efficient release of the cytokine and that IL-18 does not promote increased replication of SIV in rhesus PBMC.