The inhibition of death-receptor apoptosis is a conserved viral function. The murine cytomegalovirus (MCMV) gene M36 is a sequence and functional homologue of the human cytomegalovirus gene UL36, and it encodes an inhibitor of apoptosis that binds to caspase-8, blocks downstream signaling and thus contributes to viral fitness in macrophages and in vivo. Here we show a direct link between the inability of mutants lacking the M36 gene (ΔM36) to inhibit apoptosis, poor viral growth in macrophage cell cultures and viral in vivo fitness and virulence. ΔM36 grew poorly in RAG1 knockout mice and in RAG/IL-2-receptor common gamma chain double knockout mice (RAGγC−/−), but the depletion of macrophages in either mouse strain rescued the growth of ΔM36 to almost wild-type levels. This was consistent with the observation that activated macrophages were sufficient to impair ΔM36 growth in vitro. Namely, spiking fibroblast cell cultures with activated macrophages had a suppressive effect on ΔM36 growth, which could be reverted by z-VAD-fmk, a chemical apoptosis inhibitor. TNFα from activated macrophages synergized with IFNγ in target cells to inhibit ΔM36 growth. Hence, our data show that poor ΔM36 growth in macrophages does not reflect a defect in tropism, but rather a defect in the suppression of antiviral mediators secreted by macrophages. To the best of our knowledge, this shows for the first time an immune evasion mechanism that protects MCMV selectively from the antiviral activity of macrophages, and thus critically contributes to viral pathogenicity in the immunocompromised host devoid of the adaptive immune system.

Viral immune evasion is currently understood to focus on deflecting CD8 T cell recognition of infected cells by disrupting antigen presentation pathways. We evaluated viral interference with the ultimate step in cytotoxic T cell function, the death of infected cells. The viral inhibitor of caspase-8 activation (vICA) conserved in human cytomegalovirus (HCMV) and murine CMV (MCMV) prevents the activation of caspase-8 and proapoptotic signaling. We demonstrate the key role of vICA from either virus, in deflecting antigen-specific CD8 T cell-killing of infected cells. vICA-deficient mutants, lacking either UL36 or M36, exhibit greater susceptibility to CD8 T cell control than mutants lacking the set of immunoevasins known to disrupt antigen presentation via MHC class I. This difference is evident during infection in the natural mouse host infected with MCMV, in settings where virus-specific CD8 T cells are adoptively transferred. Finally, we identify the molecular mechanism through which vICA acts, demonstrating the central contribution of caspase-8 signaling at a point of convergence of death receptor-induced apoptosis and perforin/granzyme-dependent cytotoxicity.

CD8+ T cells are involved in the pathogenesis of inflammatory bowel disease (IBD), a complex multifactorial chronic disease. Here, we present an overview of the current research with the controversial findings of CD8+ T cell subsets and discuss some possible perspectives on their therapeutic value in IBD. Studies on the role of CD8+ T cells in IBD have contradictory outcomes, which might be related to the heterogeneity of the cells. Recent data suggest that cytotoxic CD8+ T cells (Tc1) and interleukin (IL) 17-producing CD8+ (Tc17) cells contribute to the pathogenesis of IBD. Moreover, subsets of regulatory CD8+ T cells are abundant at sites of inflammation and can exhibit pro-inflammatory features. Some subsets of tissue resident memory CD8+ T cells (Trm) might be immunosuppressant, whereas others might be pro-inflammatory. Lastly, exhausted T cells might indicate a positive outcome for patients. The function and plasticity of different subsets of CD8+ T cells in health and IBD remain to be further investigated in a challenging field due to the limited availability of mucosal samples and adequate controls.

Cytomegalovirus (CMV) is a ubiquitous beta-herpesvirus whose reactivation from latency is a major cause of morbidity and mortality in immunocompromised hosts. Mouse CMV (MCMV) is a well-established model virus to study virus-host interactions. We showed in this study that the CD8-independent antiviral function of myeloid dendritic cells (mDC) is biologically relevant for the inhibition of MCMV replication in vivo and in vitro. In vivo ablation of CD11c ؉ DC resulted in higher viral titers and increased susceptibility to MCMV infection in the first 3 days postinfection. We developed in vitro coculture systems in which we cocultivated MCMV-infected endothelial cells or fibroblasts with T cell subsets and/or dendritic cells. H uman cytomegalovirus (HCMV) is a betaherpesvirus which establishes a lifelong latent infection in immunocompetenthosts. Latent HCMV is present in the majority of people worldwide, but the primary infection is usually asymptomatic. The primary infection is well contained by the immune cells, such as natural killer (NK) cells and T cells, which also prevent viral reactivation from latency (1, 2).Their activation depends on cross talk with dendritic cells (DC) (3, 4), and this interaction plays an important role in CMV control (5-7). The direct effect of DC on viral replication remains, however, unclear.In immunocompromised hosts, like AIDS patients or people undergoing transplantation, the virus cannot be contained, and its reactivation from latency has been associated with severe disease (8). Therefore, to develop new therapeutic approaches against CMV disease, it is exceedingly important to understand the immune mechanisms that drive the virus into latency.Murine cytomegalovirus (MCMV) is a natural pathogen of the mouse. It shows numerous analogies in latency and reactivation to the human virus, and its genome displays substantial similarity to the HCMV one (9). Therefore, MCMV is a widely used model for CMV infection and immunity (10)(11)(12).During primary infection, MCMV infects various different cell types, such as macrophages and DC but also nonhematopoietic cells, including endothelial and epithelial cells (13). On the other hand, the establishment of latency appears to be restricted to certain cell types. Latent HCMV was found in blood monocytes and in progenitor cells of the myeloid lineage (14-19), whereas liver sinusoidal endothelial cells (LSEC) were shown to be a site of MCMV latency and reactivation (20, 21), although myeloid cells might also present a latent reservoir in the mouse (22, 23).

Identification and purification of antigen-specific T cells without altering their functional status are of high scientific and clinical interest. Staining with major histocompatibility complex (MHC)-peptide multimers constitutes a very powerful method to study antigen-specific T-cell subpopulations, allowing their direct visualization and quantification. MHC-peptide multimers, such as dimers, tetramers, pentamers, streptamers, dextramers and octamers have been used to evaluate the frequency of CD8(+) T cells, specific for tumor/leukemia-associated antigens as well as for viral antigens, e.g., CMVpp65 and EBV-EBNA. Moreover, MHC-peptide multimers have been used for rapid and efficient ex vivo isolation and expansion of T cells. A recent development in the field of MHC-peptide multimers led to the purification of CD8(+) T cells specific for leukemia antigens. This might help to select leukemia-specific donor lymphocyte infusions (DLIs), thus allowing dissection of the noxious graft-versus-host disease (GvHD) from beneficial anti-viral and even anti-leukemic effects. This review covers different types of MHC-peptide multimers and their applications, as well as the impact that multimers might have on further development of DLIs.