Human cytomegalovirus: a viral complication in transplantation

Human cytomegalovirus (CMV) belongs to the group of herpes viruses and is commonly found in the human population. After a primary infection. the virus is not completely eliminated from the body but remains latently present in the host. Although CMV infections are generally subclinical in healthy, immunocompetent subjects, they may give serious clinical problems in immunosuppressed patients. Organ transplant recipients who are treated with immunosuppressive agents in order to prevent graft rejection often suffer from active CMV infections. CMV infection may induce organ graft failure in solid organ transplant patients, in particular, after bone marrow transplantation CMV-induced pneumonitis is a lethal threat. Nowadays, active CMV infections can be treated efficiently with antiviral agents but long-lasting, subclinical CMV infections have been associated with the development of acute and chronic rejection. This implies that CMV can cause graft failure in the long-term. To prevent CMV disease, it is important to determine the remaining capacity of the CMV-specific immune response in the immunosuppressed subject and to define to which viral antigens this immune response is directed. When it is known which part of the immune system fails in patients suffering from CMV infections. and which viral antigens are important targets for the immune response, this may lead to adaptation of the antiviral treatment and also to the development of an effective vaccine. Here, we give an overview of the biology of CMV and the immune response to this virus. CMV is, like the other herpes viruses, widely distributed in the human population. In general, there are two preferential time periods during lifetime in which people become infected, i.e. early childhood and early adulthood (1.). The prevalence of CMV in the human population ranges from 40 to 100% depending on population density and socio-economic background. Infection during early childhood occurs from mother to infant, before or during birth and/or breastfeeding. Children also. get infected in day care centers, possibly via contact with CMV present in excreta like saliva or urine. CMV was recovered from toys mouthed by infants and toddlers (1.). Parents can be infected by their children who excrete CMV. Virus excretion in urine and saliva can persist for years after congenitally, prenatally, and early postnatally acquired infection. In early adulthood people are usually infected by sexual transmission. Iatrogenic risks of infection are associated with blood transfusions and organ transplantations. Primary CMV infections often remain asymptomatic, although they may have various clinical manifestations. After the primary infection the virus remains latently present in its host. The underlying mechanisms of latency have not been elucidated yet, and it is still not clear whether CMV is only inactively present as untranslated DNA or if it gives rise to a slowly proceeding, persistent infection. The fact that significant humoral and cellular immunity remains detectable during life favours the latter opinion. Various cell types are known to harbour the viral genome of CMV, although they seem to be nonpermissive for the virus. During normal infection resulting in active viral production (active infection) the salivary glands are an important place for growth and dissemination of CMV. During disseminated CMV infection viral antigens have been found in various organs. In lung tissue, epithelial cells and fibroblasts were the predominantly infected cells, while in gastrointestinal tissue mostly fibroblasts, endothelial, epithelial, and smooth muscle cells were infected. Infected macrophages were rarely seen (2.). All these cell types might have the capacity to harbour CMV during latency and may be the source of recurrent infections. It should be noted that CMV is a virus with low cytopathogenicity and even with generalized disease the number and distribution of infected cells in diseased organs is often not reflected by the functional disturbances. This suggests that mechanisms other than specific viral cytolysis may contribute to organ dysfunction (3.). Since CMV is transmitted with blood and organs from CMV-seropositive individuals, much of in vitro research has focussed on latency in blood cells. The general assumption concerning latency is that in latently infected cells viral DNA can be detected but no viral proteins. Accordingly, CMV DNA was shown to be present in monocytes of healthy asymptomatic subjects but not in the T cell population of these subjects (4.) nor in the polymorphonuclear cells (5.). Monocytes, positive for CMV DNA, did not produce any viral RNA, but after in vitro stimulation and differentation the monocyte-derived macrophages expressed immediate-early RNA transcripts. However, no transcripts encoding for other viral proteins could be detected, indicating that this does not lead to virus production (6.). Later studies showed, however, that monocytic cell precursors at specific developmental stages could be efficiently and productively infected in vitro with clinical isolates of CMV suggesting that in vivo monocytes may acquire CMV during their development in the bone marrow (7.-9.). In immunosuppressed patients viral DNA was found in CD34+ cells from blood and bone marrow (10.). In another study transcripts encoding for immediate early (IE) proteins were found in latently infected cells, and the striking finding was that most of these IE transcripts remained unspliced, which is in contrast to the splicing pattern that occurs in permissive cells, i.e. in fibroblasts. Eventually, most samples yielded virus (11., 12.). The function of these unspliced transcripts is still elusive, but they might be involved in the process of latency. Until now, no latency-associated genes nor latency-associated viral proteins have been identified for CMV. CMV is a strongly cell-associated virus, and dissemination of the virus is thought to occur mainly by cell-cell contact. However, the mechanisms by which CMV infects multiple organs and various cell types have not been elucidated yet. Circulating CMV-infected endothelial cells have been found in patients during active infection, and these cells have been shown to produce viral particles (13.). Monocytes are also thought to play a role in transmission since they harbour latent CMV and migrate through the body. An interesting finding has been described by Waldman et al. (14.) in which both monocytes and endothelial cells were thought to be responsible for the dissemination of CMV. They showed that monocytes could be infected when they were cocultured for 5-7 d with endothelial cells that had been infected with a clinical strain of CMV. CMV-infected T or B lymphocytes were never found. These infected monocytes showed the full range of CMV gene expression and were able to infect, in their turn, fresh, virus-free endothelial cells. These authors hypothesize that, in vivo, monocytes can be effectively infected by CMV-infected endothelial cells and in their turn spread virus infection by infecting endothelial cells in other parts of the body. In healthy individuals a balance exists between the immune system and CMV; latency is actively maintained by the immune system, and CMV-infected cells will be eliminated at an early stage of infection. However, when the immune system is compromised this balance can not be maintained and reactivation may occur. This is the case in, for example, transplant recipients and AIDS patients. After transplantation active CMV infections are very frequently seen and may cause severe clinical problems. General symptoms are spiking fever, malaise, and arthralgia. Organ involvement can lead to graft failure. CMV itself has immunosuppressive effects, since lowering of the immunosuppressive regimen during active infection does not lead to graft rejection. CMV also inhibits the IL-1 production by monocytes, and decreased in vitro proliferative responses of T cells are seen to mitogens and antigens. In transplantation, a primary infection occurs when a CMV-seronegative subject receives a graft from a CMV-seropositive donor. A secondary infection can be induced, probably as a result of the immunosuppressive regimen via reactivation of the pre-existing endogenous CMV in seropositive recipients. When reactivation of a latent infection occurs together with reinfection from the donor, CMV causes a so-called ‘superinfection‘. CMV infection has considerable cost impact on transplantations. Patients developing CMV discase are hospitalized longer during the first year than the control group. Therefore, the total costs are 2.5 times higher for patients with CMV disease (15.). Active CMV infections can be treated efficiently with ganciclovir. This is a nucleoside analog which is phosphorylated into its active form by the viral protein pUL97, so it is preferentially effective in CMV-infected cells (16.). With this potent antiviral agent life-threatening CMV disease can be prevented. However, subclinical CMV infections have been proposed to play a role in the development of transplant failure in the long run, i.e. in the induction of the chronic transplant dysfunction (CTD) syndrome. In animal models it has been proven that CMV infections enhance smooth muscle cell proliferation and intimal thickening of rat aortic allografts. Also in humans it is generally agreed that CMV is a cofactor in the development of acute rejection and CTD. CTD manifestations such as transplantation-associated atherosclerose in cardiac allografts and obliverative bronchiolitis in lung allografts, have been associated with CMV infections (17.-19.). The underlying mechanism has not been revealed yet. In vitro, data have indicated that CMV enhances MHC class I expression on neighbouring non-infected cells: When endothelial cells are infected with low doses of virus, the uninfected cells lining the CMV-infected cell show an upregulation of MHC I, possibly as a result of IFN production by the infected cell. expression is not by CMV infection Also the is on epithelial and endothelial cells after CMV infection Although MHC class is not by CMV infection of endothelial cells T cells higher class expression on endothelial cells by when they CMV-infected endothelial cells This expression of MHC may lead to the mechanisms by CMV infection may contribute or to the development of graft failure in transplant recipients. In a rat it was shown that after lung transplantation was by a viral infection, suggesting that the viral infection the chronic rejection by a To effective it is important to which proteins are for immune In the at is of the of the various CMV proteins. 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