Memory Inflation during Chronic Viral Infection Is Maintained by Continuous Production of Short-Lived, Functional T Cells

SummaryDuring persistent murine cytomegalovirus (MCMV) infection, the T cell response is maintained at extremely high intensity for the life of the host. These cells closely resemble human CMV-specific cells, which compose a major component of the peripheral T cell compartment in most people. Despite a phenotype that suggests extensive antigen-driven differentiation, MCMV-specific T cells remain functional and respond vigorously to viral challenge. We hypothesized that a low rate of antigen-driven proliferation would account for the maintenance of this population. Instead, we found that most of these cells divided only sporadically in chronically infected hosts and had a short half-life in circulation. The overall population was supported, at least in part, by memory T cells primed early in infection, as well as by recruitment of naive T cells at late times. Thus, these data show that memory inflation is maintained by a continuous replacement of short-lived, functional cells during chronic MCMV infection

Innate immunity defines the capacity of antiviral T cells to limit persistent infection

Effective immunity requires the coordinated activation of innate and adaptive immune responses. Natural killer (NK) cells are central innate immune effectors, but can also affect the generation of acquired immune responses to viruses and malignancies. How NK cells influence the efficacy of adaptive immunity, however, is poorly understood. Here, we show that NK cells negatively regulate the duration and effectiveness of virus-specific CD4+ and CD8+ T cell responses by limiting exposure of T cells to infected antigen-presenting cells. This impacts the quality of T cell responses and the ability to limit viral persistence. Our studies provide unexpected insights into novel interplays between innate and adaptive immune effectors, and define the critical requirements for efficient control of viral persistence

Protection against murine cytomegalovirus infection by passive transfer of neutralizing and non-neutralizing monoclonal antibodies

The ability of eight neutralizing monoclonal antibodies (MAbs) specific for structural proteins of murine cytomegalovirus (MCMV) to protect mice passively against MCMV infection was examined to determine firstly whether a correlation existed between the neutralization titres of the MAbs in vitro and the protection afforded by the MAbs in vivo and, secondly, the contribution of the host towards neutralization by the MAbs in vivo. The reduction in MCMV titre in the livers of BALB/c and C57BL/10 mice by the MAbs closely correlated with their neutralization titres in vitro. However, in the spleens of BALB/c mice, in which MCMV replicates to high titre, almost all of the MAbs tested were ineffective in reducing MCMV replication. Indeed, a significant increase in splenic MCMV replication was observed in mice treated 24 h prior to MCMV replication with either neutralizing MAbs or polyclonal Ig. Each of six MAbs prophylacti-cally protected between 66 and 100 % of mice from an intraperitoneal challenge with 4 LDs0 MCMV regardless of their neutralization titre in vitro. The persistence of MCMV replication in the salivary gland was not prevented by either polyelonal Ig or MAbs. Despite the absolute requirement for complement for the neutralization of MCMV in vitro, both polyclonal lg and MAb 4F9 protected A/J mice, which are deficient in the fifth component of complement, as efficiently as they did complement competent BALB/c mice. These results demonstrate that MAbs specific for single MCMV polypeptides are protective in vivo. In addition, the extent to which the MAbs protected against MCMV could not be predicted from their immunoreactive or neutralizing titres in vitro or by their effect on splenic MCMV replication in vivo. Furthermore, these studies suggest that the mechanism(s) of neutralization of MCMV in vitro are different to those which act in vivo

Molecular characterization of virus-specific RNA produced in the brains of flavivirus-susceptible and -resistant mice after challenge with Murray Valley encephalitis virus

Natural resistance to flaviviruses in mice is controlled by a single genetic locus, Flv, on chromosome 5. Although the mechanism of this resistance is not fully understood, it is believed to operate at the level of virus replication rather than the immune response. It has been hypothesized that enhanced production of viral defective interfering (DI) particles is responsible for a substantial reduction in the titres of infectious virus in resistant mice. However, this has never been established at the molecular level since such particles have not been isolated and characterized. We have studied the products of virus replication in the brains of flavivirus-susceptible C3H/HeJ (Flv(s)) and - resistant congenic C3H/RV (Flv(r)) mice after an intracerebral challenge (i.c.) with Murray Valley encephalitis (MVE) virus and have found no evidence for the accumulation of truncated viral RNA in the brains of resistant mice. All three major viral RNA species, the replicative intermediate (RI), replicative form (RF) and virion RNA (vRNA) together with a subgenomic RNA species of 0.6 kb, which has not been previously described, were present in the brains of both mouse strains. However, the viral RF and RI RNA forms preferentially accumulated in the brains of resistant mice. Thus, we confirm that the resistance allele Flv(r) interferes with discrete steps in flavivirus replication, although the precise mechanism remains to be determined

Murine cytomegalovirus strains co-replicate at multiple tissue sites and establish co-persistence in salivary glands in the absence of Ly49H-mediated competition

Infection with multiple genetically distinct strains of pathogen is common and can lead to positive (complementation) or negative (competitive) within-host interactions. These interactions can alter aspects of the disease process and help shape pathogen evolution. Infection of the host with multiple strains of cytomegalovirus (CMV) occurs frequently in humans and mice. Profound, NK-cell-mediated (apparent) competition has been identified in C57BL/6 mice, and prevented the replication and shedding of certain co-infecting CMV strains. However, the frequency of such strong competition has not been established. Other within-host interactions such as complementation or alternative forms of competition remain possible. Moreover, high rates of recombination in both human CMV and murine CMV (MCMV) suggest prolonged periods of viral co-replication, rather than strong competitive suppression. An established model was employed to investigate the different possible outcomes of multi-strain infection in other mouse strains. In this study, co-replication of up to four strains of MCMV in the spleen, liver and salivary glands was observed in both MCMV-susceptible and MCMV-resistant mice. In the absence of apparent competition, no other forms of competition were unmasked. In addition, no evidence of complementation between viral strains was observed. Importantly, co-replication of MCMV strains was apparent for up to 90 days in the salivary glands. These data indicated that competition was not the default outcome of multi-strain CMV infection. Prolonged, essentially neutral, co-replication may be the norm, allowing for multi-strain transmission and prolonged opportunities for recombination