6533b871fe1ef96bd12d26a8

RESEARCH PRODUCT

Control of murine cytomegalovirus in the lungs: Relative but not absolute immunodominance of the immediate-early 1 nonapeptide during the antiviral cytolytic T-lymphocyte response in pulmonary infiltrates

subject

description

Effective control by the immune system is a hallmark of cytomegalovirus (CMV) infection. Accordingly, human CMV disease is a medical problem restricted to the immunologically immature or immunocompromised host (for a review, see reference 21). Murine models have implicated natural killer (NK) cells and CD8 T cells in the control of CMV infection. While NK cells mediate early protection in genetically resistant mouse inbred strains (4, 5, 31, 51), CD8 T cells establish enduring protective memory and function as principal antiviral effectors in susceptible strains (31). Specifically, in the BALB/c strain, major histocompatibility complex (MHC) class I-restricted antiviral CD8 T cells resolve acute murine CMV infection and prevent lethal CMV disease (45; for a review, see reference 27). In a model of experimental bone marrow transplantation (BMT), reconstitution of CD8 T cells proved to be essential for the prevention of lethal murine CMV pathogenesis in multiple organs (36). Furthermore, preemptive experimental cytoimmunotherapy by adoptive transfer of antiviral CD8 T cells limited the burden of latent viral genome and thereby reduced the risk of virus recurrence (54). Since efficient reconstitution of CD8 T cells after clinical BMT is of positive prognostic value for a control of human CMV (46, 48), experimental BMT in the susceptible BALB/c mouse strain is likely to be a relevant model for studying the immune response to CMV in the specific context of immunological reconstitution after BMT. The established role of CD8 T cells in immunity to murine as well as human CMV contrasts with the recent finding that these viruses have both evolved manifold immune evasion mechanisms that interfere at various steps in the MHC class I pathway of antigenic peptide presentation in the infected cell (reviewed in reference 19). Downregulation of MHC class I cell surface expression should result in enhanced susceptibility to NK cells (22). Notably, by expressing the respective viral class I homologs, human as well as murine CMVs have acquired the potential to evade control by NK cells as well (14, 47). Effective in vivo control of CMV by CD8 T cells implies a leakiness of molecular immune evasion. It is a known but so far insufficiently understood phenomenon that the immune response to virus infections is often focused on a limited number of immunodominant peptides. To become immunodominant, a viral peptide must be superior to other potentially antigenic peptides in passing through all the critical steps in the pathway of antigen processing and presentation, namely, efficient generation by protein cleavage at the proteasome, transport into the endoplasmic reticulum, high-affinity binding to the presenting MHC class I molecule, and transport of the assembled MHC-peptide complex to the cell surface. Viral immune evasion mechanisms place further obstacles in the way of candidate peptides. Accordingly, an immunodominant peptide must be one that also overcomes or circumvents the evasion strategies of the virus more efficiently than others do. Therefore, immune evasion and peptide immunodominance are likely to be linked phenomena. Specifically, although the genome of murine CMV comprises ca. 170 open reading frames (37) with the capacity to encode numerous antigenic peptides for any MHC haplotype, an antigen expressed during the immediate-early (IE) phase of the viral replication cycle proved to be immunodominant in BALB/c mice (42, 43). The immunodominant antigen was identified as a nonapeptide with the sequence YPHFMPTNL, derived from the regulatory IE1 protein pp89 and presented by the MHC class I molecule Ld (13, 44). Its significance in protection against lethal murine CMV disease has been documented by the protective efficacy of a vaccinia virus recombinant expressing the IE1 nonapeptide selectively (12). Apparently, if a limited number or, in the extreme, only a single “privileged” antigenic peptide overcomes the immune evasion strategies of the virus, this will suffice for effective antiviral control by CD8 T cells. The central question of how many different viral peptides are involved in the in vivo immune response to acute CMV infection has remained unanswered to date because lymphocytes derived from lymphoid tissues did not exert an ex vivo cytolytic activity (40). Current knowledge thus rests on cytolytic T-lymphocyte lines (CTLL) propagated in culture under conditions that entail the risk of arbitrary selection. We demonstrate here that pulmonary infiltrates that develop after BMT and concurrent murine CMV infection provide a microenvironment for the differentiation of CD8 T cells into functional cytolytic T lymphocytes (CTL). This gave us for the first time the opportunity to study the specificity of CTL that are operative at a relevant organ site of CMV pathogenesis, namely, the lungs. CTL isolated from the pulmonary infiltrates were tested directly with naturally processed peptides derived from the infected lungs. The result was surprising. Although the pulmonary CTL displayed a high cytolytic activity and lysed infected target cells at all stages of the viral replicative cycle, this cytolytic activity could not be quantitatively attributed to immunodominant peptides.