Cytomegalovirus-specific CD4 and CD8 T cell responses in infants and children.

Congenital cytomegalovirus (CMV) infection is the most common congenital infection causing childhood morbidity. The pathogenetic mechanisms behind long-term sequelae are unclear, but long-standing viremia as a consequence of the inability to convert the virus to a latent state has been suggested to be involved. Whereas primary CMV infection in adults is typically rapidly controlled by the immune system, children have been shown to excrete virus for years. Here, we compare T cell responses in children with congenital CMV infection, children with postnatal CMV infection and adults with symptomatic primary CMV infection. The study groups included 24 children with congenital CMV infection, 19 children with postnatal CMV infection and eight adults with primary CMV infection. Among the infants with congenital CMV infection, 13 were symptomatic. T cell responses were determined by analysis of interferon gamma production after stimulation with CMV antigen. Our results show that whereas adults display high CMV-specific CD4 T cell responses in the initial phase of the infection, children younger than 2 years have low or undetectable responses that appear to increase with time. There were no differences between groups with regard to CD8 T cell function. In conclusion, inadequate CD 4 T cell function seems to be involved in the failure to get immune control of the CMV infection in children younger than 2 years of age with congenital as well as postnatal CMV infection.

Cellular responses to cytomegalovirus in immunosuppressed patients: circulating CD8+ T cells recognizing CMVpp65 are present but display functional impairment.

The availability of tetrameric complexes of HLA class I molecules folded with immunodominant peptides makes it possible to utilize flow cytometry for rapid and highly specific visualization of virus specific CD8+ T cells. An alternate technique is to incubate whole blood with specific antigens and to subsequently detect and characterize responding T cells (e.g. by performing intracellular staining of interferon-gamma). By using an HLA-A2 tetramer construct folded with the same immunodominant CMV-peptide as that used for peptide pulsing, we monitored both the presence and functional capacity of CMV-specific CD8+ T cells. In addition T cell activation was assayed by determination of CD38 and CD69 expression. Twelve organ transplant patients and 31 healthy blood donors with latent CMV infection were investigated using CMV pp65 tetramer staining and intracellular staining of interferon-gamma after CMV pp65 peptide pulsing or CMV lysate pulsing. CMV-specific T cells were detected in similar absolute numbers as well as frequencies of T cells in the two groups investigated. However, the CMV-specific CD8+ T cells in immunosuppressed individuals showed a decreased functional response to the CMV-peptide, as evidenced by reduced interferon-gamma production when compared to healthy blood donors (19%; 42%, P < 0.005). In addition, CD38 expression was markedly higher in immunosuppressed patients compared to healthy blood donors (24%; 6%, P < 0.005). In a case report we demonstrate that reactivation of CMV can occur in an immunosuppressed patient with high number of CMV-specific T cells, but without functional capacity. Hence, these findings reflect impaired activation of cytotoxic T cells controlling latent CMV infection in immunosuppressed patients.