Cytomegalovirus-specific T cells are primed early after cord blood transplant but fail to control virus in vivo

Abstract

A disadvantage of umbilical cord blood transplantation (UCBT) is the delay in immune reconstitution, placing patients at increased risk for infections after transplant. Cytomegalovirus (CMV) in particular has been shown to cause significant morbidity in patients undergoing UCBT. Here, we comprehensively evaluate the development of CD4(+) and CD8(+) T-cell responses to CMV in a cohort of patients that underwent double UCBT. Our findings demonstrate conclusively that a diverse polyclonal CMV-specific T-cell response derived from the UCB graft is primed to viral antigens as early as day 42 after UCBT, but these T cells fail to achieve sufficient numbers in vivo to control CMV reactivations. This is not due to an inherent inability of UCB-derived T cells to proliferate, as these T cells underwent rapid proliferation in vitro. The TCR diversity and antigen specificity of CMV-specific T cells remained remarkably stable in the first year after transplant, suggesting that later control of virus replication results from improved function of T cells primed early after transplant and not from de novo responses derived from later thymic emigrants. Ex vivo expansion and adoptive transfer of CMV-specific T cells isolated from UCBT recipients early after transplant could augment immunity to CMV.

Figure 1

CMV reactivations during the first 400 days posttransplant. The occurrence of each reactivation in each 50-day interval through day 400 after transplant is depicted by a dark circle on the timeline for the 13 of 15 CMV-seropositive patients that reactivated CMV. The number in parentheses indicates the maximum CMV DNA copy number per microliter observed for each patient. The survival (days) after transplant is shown in the column to the right of each patient timeline.

Figure 2

Recovery of lymphocytes and T-cell subsets after UCBT. The absolute number of lymphocytes, and CD3⁺, CD4⁺, and CD8⁺ T cells in the peripheral blood at intervals after UCBT is shown. The dashed line in each graph indicates the lower level of normal for each cell subset.

Figure 3

Detection of IFN-γ⁺ CMV-specific CD8⁺ T cells after UCBT by CFC of PBMCs and after in vitro expansion of T cells. (A) PBMCs from a normal donor (ND) and from 2 representative CMV-seropositive UCBT recipients were analyzed for CMV-specific CD8⁺ T cells by CFC. The number in the top right quadrant indicates the percentage of IFN-γ⁺ T cells in the CD8⁺ subset. (B) CD8⁺ T-cell lines were generated from PBMCs obtained at the indicated times by a single in vitro stimulation with RV798-infected fibroblasts and then assayed for IFN-γ by CFC. Data are shown for a normal donor and a representative UCBT patient at multiple time points. Mock-infected autologous fibroblasts served as a negative control in the assay. (C) IFN-γ CFC assay for CMV-specific T cells in T-cell lines from each of 19 UCBT recipients at intervals after transplant (15 CMV positive and 4 CMV negative) and from 2 normal donors. The data shows the percentage of IFN-γ⁺ T cells in the gated lymphocyte population after restimulation with RV798-infected fibroblasts. White bars, day 56; light gray bars, day 80; dark gray bars, day 180; black bars, day 365; asterisk (*) indicates not done. (D) Lysis of autologous fibroblasts infected with RV798 (light gray bars), mock-infected (white bars), or HLA-mismatched fibroblasts either RV798-infected (dark gray bars) or mock-infected (black bars) by CMV-specific T-cell lines from UCBT recipients. The graph shows the mean specific lysis of cell lines from different time points after UCBT in 5 patients. The effector-to-target (E:T) ratio was 10:1.

Figure 4

Expansion of CMV-specific CD4⁺ and CD8⁺ T cells from UCBT recipients and characterization of CD8 T-cell specificity. (A) Frequency of CMV-specific CD8⁺ T cells by CFC for IFN-γ performed on 2 representative T-cell lines expanded after IFN-γ capture. (B) CMV-specific T cells derived from UCBT recipients recognize pp65 peptides. Polyclonal T-cell lines were incubated with a peptide pool of 15-mer peptides with 11 amino acid overlaps spanning the entire pp65 protein and evaluated by CFC to detect IFN-γ⁺ cells. Results of 5 patients and 1 ND are shown. (C) Viral genome scan to determine the antigen specificity of CD8⁺ T cells derived from UCBT recipients at different times after transplant. Polyclonal T-cell lines were incubated with COS cells that were transfected without or with a plasmid encoding a patient specific HLA DNA alone (gray bars), with a plasmid encoding a single CMV-ORF alone (white bars) or cotransfected with both plasmids encoding the CMV ORF and the HLA allele (black bars). Supernatants were collected at 24 hours and assayed for IFN-γ by enzyme-linked immunosorbent assay. Results for the genome scan for T-cell lines derived from blood obtained on day 80, 180, and 365 post-UCBT from 2 representative patients are shown. (D) Deep sequencing of TCR Vb receptor genes in CMV-specific polyclonal T-cell lines from UCBT recipients (patient no. 2 top graph and patient 4 bottom graph) was performed. Results show the percentage of total clones of the top 10 clones tracked over time in both patients.