Int. I presentation of IE1-pp72-derived peptides. We show that the amount of incoming pp71 positively correlates with IE1-pp72 protein levels and with the presentation of IE1-derived peptides. This indicates that the amount of the IE1 protein, induced by pp71, rather than a putative immunoevasive function of the tegument protein, determines MHC class I antigen presentation of IE1-derived peptides. This process proved to be independent of the presence of pp65, which had been reported to interfere with IE1 presentation. It may thus be beneficial for the success of HCMV replication to limit the level of pp71 delivered from infecting particles in order to avoid crucial levels of MHC class I presentation of IE protein-derived peptides. INTRODUCTION Infection with human cytomegalovirus (HCMV) is usually tightly controlled by the immune system. Only immature or Dehydrocholic acid compromised immune defense functions may allow viral Dehydrocholic acid replication to proceed to an extent that results in overt clinical disease. Cytomegaloviruses likely would have been eradicated if it was not for the expression of immunoevasive Dehydrocholic acid functions that antagonize intrinsic, innate, and adaptive responses in order to allow the establishment of latency (1C4). CD8 T cells are particularly powerful in restricting both acute contamination and viral reactivation (5C7). Glycoproteins encoded by the US2-11 gene region efficiently disturb the major histocompatibility complex (MHC) class I antigen presentation pathway, thereby antagonizing CD8 T cell control (8C11). Expression of these genes in a rhesus cytomegalovirus model prevented superinfection in seropositive animals (12). Besides gpUS2-gpUS11, the tegument proteins pp65 and pp71 have also been suggested to interfere with MHC class I-mediated antigen presentation (13, 14). Although considerable knowledge has been gathered about the regulation of the MHC class I presentation pathway by studying HCMV immunoevasive proteins, there is still only limited information available on how single evasins take action in the context of contamination (8, 9). One question of particular interest is Dehydrocholic acid usually how MHC class I antigen presentation was suppressed at the immediate early (IE) stages of contamination. Abundant expression of the IE proteins IE1 (IE1-pp72) and IE2 (IE2-pp86), a crucial event for efficient progression Rabbit Polyclonal to Merlin (phospho-Ser10) of lytic contamination, relies on pp71-mediated derepression of the major IE enhancer-promoter (15C18). On the other hand, IE1 and IE2 are major target antigens of the CD8 T cell response against HCMV (19C21). Presentation of antigenic peptides from the two proteins by MHC class I at IE phases may be detrimental to the success of viral contamination. Mechanisms that suppress presentation are likely implemented soon after contamination, thereby preventing acknowledgement by cytotoxic CD8 T lymphocytes (CTL) and cell lysis. The gpUS3 has been reported to be expressed at IE occasions (22C26). However, it may have only limited impact on IE1 peptide presentation (27). We have recently shown that both gpUS2 and gpUS11 are also expressed at IE occasions, and both proteins interfere significantly with IE1 peptide presentation by MHC class I (27). All these evasive proteins are, however, synthesized concomitantly with IE1 and IE2. Thus, their kinetics of expression may just be insufficient to prevent presentation of IE1 and IE2 early in lytic contamination. In contrast, tegument proteins, delivered to the cell via incoming viral particles, are directly available for suppression of antigen presentation. Using an HCMV deletion mutant as well as vaccinia computer virus constructs, a selective effect of pp65 on presentation of peptides from IE1 has been reported (13). The role of incoming pp71 in the suppression of CD8 T cell acknowledgement of HCMV-infected cells has not been fully elucidated. In glioblastoma cells, pp71 appeared to have an MHC class I-suppressive effect only after its synthesis (14). It remained unclear whether pp71 has an impact on MHC class I expression in permissively infected human fibroblasts and whether, in these cells, incoming pp71 would have an effect around the presentation Dehydrocholic acid of IE protein-derived peptides. The pp71 protein is a key regulator of lytic HCMV contamination (16, 28). Transcription of IE genes of HCMV critically depends on pp71, originating from infecting viral particles (4, 28). Deleting the gene encoding pp71 from your viral genome results in mutants with severely impaired growth properties (15). This renders investigations of pp71-mediated effects on MHC class I in the course of contamination complicated. We thus focused our analysis of the immunoevasive impact of pp71 on HCMV mutants that expressed various levels of pp71. We found that increased amounts of pp71 in viral particles, resulting from enhanced expression by mutant viruses, correlated with increased presentation of IE1-derived peptides by MHC.