Cytomegalovirus Reactivation Shapes NK Cells Diversity Following Allogeneic Hematopoietic Stem Cell Transplantation

Natural killer (NK) cells are the first lymphocyte population to reconstitute following allogeneic hematopoietic stem cell transplantation (HSCT) and are key players in immune defense against viral infections and malignant transformation. NK cell numbers generally recover within the first month post-transplant, but the acquisition of mature NK cell phenotype and full functional competency can take over 6 months and is influenced by various host and donor factors. Cytomegalovirus (CMV) infection has been shown to modulate NK cell maturation after HSCT.

The diversity of the NK cell repertoire is dictated by a variety of combinatorially expressed activating and inhibitory receptors that dictate the NK activation status. Moreover, whereas the expression of inhibitory receptors is primarily genetically determined, environmental factors such as viral infections influence the expression of activating receptors to a great extent.. We propose that assessment of diversity could provide a different perspective for the evaluation of the NK cell compartment after HSCT, since it is a quantitative measure that takes into account both the number and evenness of the different NK subpopulations.

To better understand the factors that influence NK cell recovery after cord blood (CB) transplant (CBT) and specifically the influence of cytomegalovirus (CMV) infection on NK cell maturity, we used 40-parameter mass cytometry (CyTOF) to interrogate the NK cell repertoire. A panel including 37 monoclonal antibodies was designed to recognize NK cells lineage markers and receptors as well as intracellular markers such as transcription factors and adaptor proteins. We first evaluated and compared the diversity of NK cells in 10 CB units and peripheral blood (PB) from 20 healthy donors. We then examined the diversity of NK cells before and after CBT in 22 serially collected blood samples from in 10 CBT recipients.

NK cell diversity was significantly lower in CB (mean 574, range 417-891) compared to PB samples from healthy donors (mean 3792, range 1284-5079; P=0.001), indicating less diversification within the naive CB NK compartment. After CBT, NK cell diversity was lower at earlier time point (Day30) (mean 1129, range 428-1768) compared to PB from healthy donors; P=0.01. The diversity of NK cells increased gradually over time following CBT (Day 30 mean 1129 range 428-1768; Day 60, mean 1185, range 515-1864; 4 months, mean 1711 range 597-2640).

We also compared the diversity of NK cells in the PB of healthy CMV seronegative (n=10) and seropositive adult donors (n=10). The diversity of NK cells was higher in CMV seropositive vs. CMV seronegative healthy donors (3887 vs 2473; P=0.04). This difference in NK diversity was even more pronounced within the KIR positive (mean 1701, range, 981-2152) compared to the KIR negative subset (mean 551, range 456-647; P=0.02), indicating that CMV infection increases the richness of mature NK cells. In keeping with these findings, CMV infection after CBT was associated with a significantly greater diversity of NK cells, especially within the KIR positive compartment (mean 604, range 207-1035) compared to the KIR negative subset (mean 283, 257-457; P=0.025). However, in CMV negative patients, we found no difference in diversity within the KIR positive and negative subsets (mean 1120 vs. 1366; P=0.28). Taken together, these data suggest that NK cell diversity reflects NK cells differentiation and maturation, and that CMV shapes NK cell diversity, especially within the KIR positive compartment.

To further understand how CMV influences NK cells diversity, we examined the top 15 NK cell subsets and their distribution at multiple timepoints before and after CMV reactivation post-CBT. CMV infection post-CBT was associated with a significant change in the distribution of NK subsets within the KIR positive population, with the top 15 subsets prior to CMV reactivation being mostly replaced by the emergence of new subsets. In contrast, the top 15 subsets within the KIR negative NK population remained stable. These data suggest that CMV drives NK cell maturation by differentiating KIR positive NK cells.

In summary, we used high-dimensional single-cell data to evaluate NK cell reconstitution following HSCT. These data can help us better understand the biology of NK cell recovery after HSCT and discover the functional significance of NK cell diversity in the setting of viral infections.