Cellular immune responses against human parvovirus B19 infection

Abstract

The role of cellular immune responses in the control of Parvovirus B19 (B19) infection has not previously been investigated. B19 does not normally cause severe clinical problems but complications may arise in immunosuppressed patients and pregnant women. Neutralizing antibodies are assumed to mediate clearance of the infection but a subset of B19 infected seropositive patients retain the infection for years.

The main focus of this thesis is Tlymphocyte responses against B19 in acute and persistent infection. In our initial study (paper I) we longitudinally analysed the B19-specific CD8+ T-cell responses in acutely infected individuals. A striking CD8+ T-cell response was observed, which was sustained and increased for many months after the resolution of the symptoms. The results indicated that CD8+ T-cell responses play a role in controlling B19 infection and the high numbers of aB19specific cells detected long after resolution of clinical symptoms demonstrated a viral-antigen drive for long periods of time.

Following our findings we aimed to characterize and define the frequency, phenotype and evolution of B19 specific CD8+ T-cells during primary B19 infection and follow-up (paper II) by using class I multimers on cells derived from patients with acute B19 infection and healthy seropositive controls. We observed that B19-specific T-cells developed and maintained an activated CD38+ phenotype with strong expression of perform and CD57. These cells were also CD27 and CD28 down-regulated. Our results showed that B19, assumed to generate an acute and lytic infection, induced a persistent antigen specific CD8+ T-cell response. The most plausible explanation for this is long-term lowlevel antigen stimulation. Having this information and technology we aimed to study the role of CD8+ T-cells in the pathogenesis of B19 infection and the mechanism of the absence of viral clearance in a group of persistently B19 infected individuals with long term B19 DNA in bone marrow (paper III).

By comparing the CD8+ T-cell responses of this group and a healthy seropositive group, we observed that the persistently B19 infected individuals showed a larger number of responses to the viral proteins, whereas responses to the non-structural protein were of lower magnitude as compared to the healthy seropositive individuals. These observations provided the first evidence of immunological discrepancies between individuals with B19 persistence and healthy individuals, which may reflect both failed immunity and antigenic exhaustion. In our fourth paper (paper IV) we developed and optimized a new method for detection of B19 viral load. Here we observed that B19 could be detected up to 2.5 years after acute infection. Only one individual in this study cleared the infection almost 2 years post-resolution of symptoms. Furthermore we characterized the cytokine profile in acute B19 infection and determined whether an imbalance of the response predominates in persistent infection (paper V).

An elevated level of Th1-type cytokines IL-2, IL-12 and IL-15 correlated with the initial peak of B19 viral load during acute infection but in the absence of IFNgamma and Th2-type cytokine response. These results are consistent with a previously described sustained CD8+ T-cell response and viremia. Persistently infected patients did not show any signs of general immunodeficiency and did not exhibit an apparent imbalance of their cytokine pattern except for an elevated IFNgamma response.

In conclusion by studying the B19specific cellular immune responses we have observed a strong and sustained CD8+ T-cell response which possesses an effector function as a result of continuous low grade antigenic stimulation for several years post onset of symptoms.