Long-Term Stable Mixed Chimerism in Patients Undergoing HSCT for Non-Malignant Disorders

Introduction. Long-term stable mixed chimerism (MC) is rare after allogeneic hematopoietic stem cell transplantation (HSCT). MC is defined as 5-95% residual recipient hematopoietic cells beyond one-year post-HSCT. The mechanisms directing hematopoietic recovery to MC in some patients and to full donor chimerism (DC) in others are poorly understood.

Methods. We compared patients with DC and long-term stable MC for a median of 9.5 years post-HSCT.  Both the clinical situation and the immune system (functional and phenotypic properties) were investigated by questionnaires, western blot, multiplex bead-based immunoassay, ELISA and flow cytometry. Blood samples were drawn at 2 weeks and >5 years post-HSCT. Additionally, lineage-specific chimerism status in the MC patients was determined. The results were analyzed by the Mann-Whitney U test, Fisher’s exact test and Spearman's rank correlation coefficient.

Results. At two weeks post-HSCT cytokine levels did not differ between the groups. Neither did the groups differ in long-term outcome in terms of general wellbeing, immunoglobulin response to vaccinations and in the most central phenotypic features of the immune system (e.g., differentiation status, CD4/CD8 ratio, B and NK cell frequency). More than 5 years post-HSCT, MC patients had significantly higher NKT cell levels (CD94+CD8+ and CD56+CD8+ T cells) than DC patients. In depth analysis of the MC patients revealed that recipient chimerism could be observed in a large variety of immune cell lineages (i.e., total, CD4+, CD8+ and γδ T cells; B cells; NK cells; myeloid cells; and cytokine producing cells). Furthermore, the recipient chimerism in different cell lineages were positively correlated with each other, e.g., an increase in CD3 chimerism was associated to an increase in CD56 chimerism. Both recipient and donor derived cells in MC patients produced cytokines to the same degree in response to mitogen stimulation. No difference in mitogen response was observed between the groups.

Discussion. Long-term MC does not appear to negatively affect the wellbeing of the patients. It could even be argued that the recipient system has a more differentiated phenotype than the donor system, indicating that MC might be of benefit protecting against infections. The recipient immune system may still be functional in MC patients, as both recipient and donor derived cells responded to mitogens. Two scenarios could explain such a symbiotic relationship between donor and recipient system: a superfluous duplicity of the entire immune system or an adjuvant role by the recipient system to the donor system.