Impact of viral infection on post-HSCT mixed T-cell chimerism in pediatric patients receiving alemtuzumab conditioning

Alemtuzumab conditioning for allogeneic hematopoietic stem cell transplantation (allo-HSCT) patients with non-malignant diseases is effective in preventing both acute and chronic graft-versus-host disease (GvHD) when it is added it to the stem cell infuscate to deplete donor T cells.

There are other chemotherapeutic drugs also used to induce T-cell depletion in allo-HSCT recipients. Regardless, some autologous T cells escape depletion after allo-HSCT and may contribute to T-cell chimerism pattern after transplant. The reasons for, and consequences of, T-cell chimerism are not fully understood.

Recently, the relationship between mixed T-cell chimerism and viral infection was discussed by Nadaf et al. at the 47th Annual Meeting of the European Society for Blood and Marrow Transplantation.¹ Their study aimed to identify the influence of virus infection on post-HSCT, lineage-specific (CD3/CD15) leukocyte mixed chimerism.

Patient characteristics

Patient selection was based on children with genetic diseases, aged 1–16 years, receiving alemtuzumab conditioning in donor (family and unrelated) marrow transplant, monitored with lineage-specific chimerism.

The demographic and clinical characterization of patients and donors are given in Table 1. Briefly, a total of 130 allo-HSCT-treated patients were selected, where 107 were engrafted, 12 experienced graft loss, and 11 patients died.

Table 1. Characteristics of patients and donors*

Key outcomes

The mean follow-up period for the study was 2.8 years post-transplant. The authors reported the following key results.¹

1. Overall survival and event-free survival at 6 years post-transplant were 91.1% and 81.5%, respectively. There was no significant difference in event-free survival for early donor T-cell chimerism in groups with less than vs more than 50% chimerism at 3 months, indicating that early mixed T-cell chimerism does not predict for subsequent graft loss.
2. There was no Grade 3/4 acute GvHD and no cases of chronic GvHD.
3. Transplant characteristics, such as age of transplant, primary diagnosis, donor type, HLA disparity, stem cell source, or intensity of conditioning, did not predict for chimerism outcomes and hence graft outcome in univariate analysis.
4. Patterns of T-cell engraftment with time revealed that initially, donor CD3 chimerism values were low, but gradually increased over time in children. Mean T-cell chimerism values were 38.69%, 58.13%, and 62.21% at 3, 6, and 12 months, respectively.
5. CD3 vs CD15 in mixed whole blood chimerism showed a mean T-cell chimerism (CD3) of 20.28% and a myeloid donor chimerism (CD15) of 99.60% (p < 0.0001) at 3 months post-transplant. T-cell chimerism increased to 40.55% (p < 0.0001), while myeloid chimerism remained at 97.34% at 6 months post-transplant, indicating that in most patients with mixed chimerism, the myeloid component remained nearly full donor. Therefore, expansion and persistence of autologous T cells was not due to allo-reactive T cells.
6.  Furthermore:
   • About 64% of patients in the mixed T-cell chimerism group had cytomegalovirus (CMV)-positive serology compared with only 41% in the complete chimerism group at 3 months post-transplant – the trend was similar after 6 months (p < 0.05).
   • In children with CMV viremia, mixed chimerism was more common at 3 and 6 months (39% and 47%) compared to those without (20% and 24%).
7. CMV infection induced stronger CD8 T-cell reconstitution, with higher numbers of CD8 T cells in the CMV viremia-positive group vs the CMV viremia-negative group (mean CD8 T-cell count per 10⁶ cells/L: 548 vs 77 at 3 months, p = 0.0001; 780 vs 244 at 6 months, p < 0.0001).

Conclusion

Conditioning with alemtuzumab yields exceptionally low rates of acute GvHD and chronic GvHD after allo-HSCT. Mixed T-cell chimerism is strongly linked with CMV viremia and positive CMV serology. Expansion of autologous virus specific CD8 T cells and the resulting mixed T-cell chimerism does not impact outcome in allo-HSCT recipients.

Therefore, only myeloid chimerism should be used to assess stem cell engraftment and graft function.