Control Of Cytomegalovirus Reactivation Post Stem Cell Transplantation Is Mediated By Simultaneous Reconstitution Of CD4+ and CD8+ CMV-Specific T Cells

Haematopoietic stem cell transplant (HSCT) patients commonly suffer from Cytomegalovirus(CMV) reactivation due to the eradication and delayed reconstitution of CMV-specific T cell immunity resulting from T cell depletion and conditioning chemotherapy. Our knowledge of T cell immunity and in particular of antigen specific CD8+ T cell responses has advanced rapidly following the introduction of HLA- class I tetramers which enables the direct enumeration and characterisation of CMV-specific CD8+ T cells. In order to study the broader CMV specific T cells reconstitution post HSCT, we used a novel HLA-class II tetramer to monitor the reconstitution of CD4+ T cells specific to a CMV-derived peptide restricted to HLA-DRB1*0701, in parallel with different HLA class I tetramers identifying CMV specific CD8+ T cells. We analysed longitudinally the immune reconstitution of a cohort of thirteen HLA-DRB1*0701 –matched HSCT patients treated for haematological malignancies and who were at high risk of CMV reactivation where both donors and recipients were CMV seropositive. Twelve received reduced intensity conditioning and T-cell depletion with in vivo Alemtuzumab, and one had non-T cell depleted myeloablative conditioning. Twelve out of 13 longitudinally studied HSCT patients experienced CMV reactivation which included multiple episodes of viremia in 8 out of 12 .The viremia resolved in less than a month of onset in 8 patients where rapid expansion of CMV specific CD4+ and CD8+ T cells was observed in response to onset of viral reactivation. In four patients, late reconstitution of CMV-specific CD4 and CD8 T cells was observed between three and six months post HSCT; these patients suffered from prolonged and multiple episodes of CMV reactivation. In reconstituting patients, there was a considerable increase in the number of CMV-specific CD4+ and CD8+ T cells, from undetectable levels before viral reactivation up to 50x10^3 cells/ml and 370x10^3 cells/ml, respectively. CMV-specific CD4+ and CD8+ T cells expanded in parallel and statistically significant correlation between these cells were observed((p=0.06)). The patient treated with myeloablative conditioning chemotherapy retained considerable numbers of (CMV-specific CD4 cells (28.7x10^3 cells/ml) at four weeks post HSCT and did not have CMV reactivation. Phenotypic analysis showed that CMV specific CD4+ T cells were predominantly effector memory cells (CCR7-CD45RA-) whilst CD8+ T cells were predominantly effector memory/CD45RA+ cells. The majority of CMV specific T cells expressed CD57 molecule and we documented a strong correlation between expansion of specific CD4+ T cells and generation of CD4+CD57+ cells post HSCT. Both CD4 and CD8 T cells specific to CMV appear to be required for the control of viremia. The use of HLA class I and class II tetramers in combination with antibodies against surface markers such as CD57 provides a broader picture of the global T cell immune response to CMV and may inform on clinical outcome and treatment guidance.