Studies of specific immunity against viral infections in patients after stem cell transplantation

Abstract: Recipients of allogeneic hematopoietic stem cell transplantation (allo-HSCT) experience a prolonged period of immune deficiency, resulting in significant morbidity and mortality from the infections. Viruses such as cytomegalovirus (CMV) and influenza virus are frequent causes of infectious complications following allo-HSCT. CMV is one of the most frequent pathogens causing life-threatening complications after HSCT. Factors like patient age, donor and recipient serostatus, type of conditioning, grade of graft versus host disease (GVHD), CMV viremia and number of CMV reactivations are all factors influencing virus reactivation and contributing to the development of the disease. Although CMV infection can be controlled by pre-emptive treatment based on sensitive diagnostic tests, there is a risk of unnecessary over-treatment, resulting in development of antiviral resistance and late onset of CMV disease. In our two CMV studies we aimed in paper I to use the immunological monitoring of CMV-specific responses to correlate transplantation factors to the recovery of cellular immunity in HSCT recipients, intending to understand how those factors influence the CMV-specific T cell reconstitution. The aim of paper II was to use the CMV-specific immunological monitoring to target antiviral therapy to patients at the highest risk of developing CMV disease. By correlating the intracellular interferon-gamma (IFN-γ) production by CMV-specific T lymphocytes with transplant factors (paper I) we observed that (a) CMV-specific T cell response is associated with a lower rate of CMV replication; (b) reduced intensity conditioning results in improved early T cell reconstitution; and (c) the recovery of CMV-specific immunity might be delayed in patients with CMV disease. Using the same method of monitoring CMV-specific immunity in the second study (paper II) we found that 25% of patients with late CMV DNAemia, who had CMV-specific immunity and lacked other risk factors of CMV disease, could be spared pre-emptive therapy without developing CMV disease or need of anti-viral therapy. Annual vaccination is the main way to prevent influenza and its complications, but it is less effective in immunocompromised patients compared to healthy individuals. Since most previous studies of vaccination efficacy investigated the humoral immune response, we aimed to develop an Elispot technique for measuring the influenza-specific cellular response as a marker of vaccine responsiveness (paper III). We measured the IFN- γ production by T cells pulsed with influenza peptides in healthy volunteers and HSCT recipients. Influenza vaccination elicited strong cell-mediated immune response in the group of healthy volunteers and we concluded that Elispot is a sensitive and specific assay for measuring cell-mediated immunity to vaccination. To further explore the responses to vaccination, we applied two Elispot methods to characterize both the cell-mediated and humoral responses to influenza vaccination (study IV). The cell-mediated responses were strong both in the healthy volunteers and in the transplanted patients. However, we found a big difference in the number of influenza-specific antibody secreting cells between healthy controls and HSCT patients, both before and after vaccination, supporting the previously shown weak ability of transplanted patients to respond to vaccination with a protective antibody response. In conclusion, routine immunologic monitoring will be helpful in guiding virological monitoring and therapeutic decisions in HSCT recipients. The inactivated split influenza vaccination elicits cell-mediated and humoral immune responses in HSCT recipients. In spite of that, the effectiveness of the vaccination in immunocompromised patients is low, which highlights the need of more immunogenic vaccine formulations for this population.