Anti-Viral Immunity to CMV Reactivation in Allogeneic HSCT Recipients.

Opportunistic CMV infections remain a major cause of morbidity and mortality in allogeneic hematopoetic stem cell transplant (HSCT) recipients, with prolonged periods of viremia associated with cytopenia and life-threatening end-organ infections. Although CMV infection in allogeneic HSCT recipients may be controlled using anti-viral drugs, re-occurrence of viremia when drug therapy is stopped is common, and the emergence of drug resistant viral strains may render the anti-viral agents ineffective. The lack of functional anti-viral immunity that leads to CMV reactivation in immunocompromized HSCT recipients is a complex phenomena and may be due to 1) failure or insufficient donor/host innate immunity to prevent reactivation of CMV from their latent state, 2) failure to effectively process and present CMV antigens by the appropriate APCs to anti-viral T cells due to conditioning regimens used to prevent graft rejection and graft-vs-host disease (GvHD); 3) lack of response of CMV specific memory T cells to control CMV infection. Lack of any or all of these three elements of immune responses are hypothesized to be responsible for clinical CMV reactivation or infections in allogeneic transplant recipients. Determining which of these three elements of effective anti-viral immunity are most critical to preventing CMV infection would help guide novel therapies designed to prevent or treat opportunistic viral infections of high-risk patients.

Emory is the core immune-monitoring laboratory for multi-center clinical study CTN 0201. As part of this study, we have samples from 69 HLA-A*2+ patients who received myeloablative conditioning and allogeneic HSCT from unrelated donors. HLA-A*2 restricted HCMV peptide pp65 (495-503, NLVPMVATV) specific tetramer+ CD8+ T-cells were determined from the blood samples obtained on 3, 6, and 12 months post transplant. Blood samples from three patients (2 HLA-A*2+ and 1 HLA-B*7+) treated at Emory had simultaneous data on both CMV viral load (measured by RT PCR) and the anti-CMV specific T-cells using the HLA-A*2 and HLA-B*7 [CMV pp65(NLVPMVATV)] tetramer reagents.

Out of 69 patients, 5% had more than 1% CD8+ T-cells expressing the HLA-A*2 restricted CMV pp65 specific T-cell receptor at 3 months post-transplant, with an increasing proportion of patients showing the presence of at least 1% CMV-specific T-cells at 6 and 12 months post-transplant. Serial simultaneous measurements of CMV viral load and anti-CMV-specific T-cells are shown in Table 1 for 3 pts treated at Emory. In each case, evidence of active CMV replication was accompanied by a rapid expansion of CMV pp65-specific T-cells in the blood. Contraction of the CMV-specific T-cell compartment subsequently occurred, in conjunction with antiviral therapy and successful clearance of the virus. CMV reactivation events were not associated with major changes in the numbers of CD4+ T-cells, monocytes, or granulocytes.

Successful control of CMV reactivation is associated with a rapid and specific CD8-mediated immune response, which works in conjunction with antiviral therapy to limit viral replication. We would predict that the inability to control CMV reactivation, which is observed in a subset of pts following allogeneic transplantation, may not be associated with the same degree of target-specific immune expansion. This may be due to low absolute T-cell numbers, lack of innate immune responses or suppression of the ability of memory T cells to become effector T cells. Ongoing analyses will correlate measurements of tetramer+ T-cells in the 0201 dataset with laboratory evidence for CMV viremia and clinical infection.

No relevant conflicts of interest to declare.