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African green monkeys (genus Chlorocebus) can be infected with species-specific simian immunodeficiency virus (SIVagm) but do not develop AIDS. These natural hosts of SIV, like sooty mangabeys, maintain high levels of SIV replication but have evolved to avoid immunodeficiency. Elucidating the mechanisms that allow natural hosts to coexist with SIV without overt disease may provide crucial information for understanding AIDS pathogenesis. Here we show that many [CD4.sup.+] T cells from African green monkeys downregulate CD4 in vivo as they enter the memory pool; that downregulation of CD4 by memory T cells is independent of SIV infection; that the [CD4.sup.-] memory T cells maintain functions that are normally attributed to [CD4.sup.+] T cells, including production of interleukin-2 (IL-2), production of IL-17, expression of forkhead box P3 and expression of CD40 ligand; that loss of CD4 expression protects these T cells from infection by SIVagm in vivo; and that these [CD4.sup.-] T cells can maintain major histocompatibility complex class II restriction. These data show that the absence of SIV-induced disease progression in natural host species may be partially explained by preservation of a subset of T cells that maintain [CD4.sup.+] T cell function while being resistant to SIV infection in vivo., SIV belongs to the group of lentiviruses that infect nonhuman primates. The lentiviruses that cause immunodeficiencies in humans and Asian macaques originated from cross-species transmission of viruses that naturally infect [...]

African green monkeys (genus Chlorocebus) can be infected with species-specific simian immunodeficiency virus (SIVagm) but do not develop AIDS. These natural hosts of SIV, like sooty mangabeys, maintain high levels of SIV replication but have evolved to avoid immunodeficiency. Elucidating the mechanisms that allow natural hosts to coexist with SIV without overt disease may provide crucial information for understanding AIDS pathogenesis. Here we show that many CD4+ T cells from African green monkeys downregulate CD4 in vivo as they enter the memory pool; that downregulation of CD4 by memory T cells is independent of SIV infection; that the CD4− memory T cells maintain functions that are normally attributed to CD4+ T cells, including production of interleukin-2 (IL-2), production of IL-17, expression of forkhead box P3 and expression of CD40 ligand; that loss of CD4 expression protects these T cells from infection by SIVagm in vivo; and that these CD4− T cells can maintain major histocompatibility complex class II restriction. These data show that the absence of SIV-induced disease progression in natural host species may be partially explained by preservation of a subset of T cells that maintain CD4+ T cell function while being resistant to SIV infection in vivo. [ABSTRACT FROM AUTHOR]

Dengue virus is the causative agent of dengue fever and the more-severe dengue hemorrhagic fever (DHF). Human studies suggest that the increased risk of DHF during secondary infection is due to immunopathology partially mediated by cross-reactive memory T cells from the primary infection. To model T cell responses to sequential infections, we immunized mice with different sequences of dengue virus serotypes and measured the frequency of peptide-specific T cells after infection. The acute response after heterologous secondary infections was enhanced compared with the acute or memory response after primary infection. Also, the hierarchy of epitope-specific responses was influenced by the specific sequence of infection. Adoptive-transfer experiments showed that memory T cells responded preferentially to the secondary infection. These findings demonstrate that cross-reactive T cells from a primary infection alter the immune response during a heterologous secondary infection. [ABSTRACT FROM AUTHOR]

Cytotoxic T lymphocytes (CTL) are hypothesized to play a role in clearance during primary dengue virus (DENV) infections, and contribute to immunopathology during secondary heterologous infections in humans. We previously reported skewed T-cell responses to secondary DENV infection in BALB/c (H-2(d)) mice, reproducing characteristics of human DENV infection. To set the stage for using widely available transgenic and knockout mice, we extended these studies to identify DENV-specific T-cell responses in C57BL/6 (H-2(b)) mice. We identified dominant CD8+ T-cell responses to H-2D(b)-restricted epitopes on the DENV NS4a (aa 249-265) and NS5 (aa 521-537) proteins. High frequencies of IFN-γ- and TNF-α-producing T cells directed at both epitopes were detected following primary infection with all four DENV serotypes, and were augmented by secondary DENV infections. In vivo cytotoxicity assays demonstrated rapid clearance of target cells pulsed with the NS4a peptide; in contrast, NS5 peptide-pulsed target cells were poorly cleared in vivo. These data characterize two H-2(b)-restricted T-cell epitopes displaying divergent in vivo function. These results should facilitate further studies of the in vivo effects of DENV-specific T cells, including the use of genetically modified mouse strains.

Secondary dengue virus (DENV) infection is a major factor contributing to the risk for severe disease, an effect that depends upon the sequence of infection with different DENV serotypes. We previously reported sequence-dependent effects of secondary DENV infection on CD8+ T-cell responses in mice. To further evaluate the effect of infection sequence, we analyzed DENV-specific CD4+ T-cell responses and their relationship to the CD8+ T-cell response. Serotype cross-reactivity of CD4+ T-cell responses also depended upon the sequence of serotypes in this model. Furthermore, adoptive transfer of memory CD4+ T cells altered the response of memory CD8+ T cells to secondary infection. These data demonstrate the interaction of different components of the T-cell response in determining the immunological outcome of secondary DENV infection.