Characterisation of Naive Immunological Profile of Umbilical Cord Blood: The Role of Mucosal-Associated Invariant T Cells and Functional Defects of T Cells and Natural Killer Cells

Patients undergoing allogeneic haematopoietic stem cell (HSC) transplantation are susceptible to infection and reactivation of viruses, such as cytomegalovirus, due to impairment of the T cell compartment following intensive immunosuppressive therapy. There is a growing interest in evaluating cell-mediated immunity to improve the management of these infections. Over the last two decades, umbilical cord blood (UCB) has emerged as an alternative source of cellular products for the treatment of a range of haematological disorders. UCB HSCs have a number of distinguishing characteristics which differentiate them from adult HSCs, including increased proliferation and increased in vitro colony forming capacity. The use of UCB as HSC grafts for allogeneic stem cell transplant has a number of advantages, including immediate availability and reduced risk of graft-versus-host disease despite donor-recipient HLA disparity. However, the limited number of HSCs present in a single UCB collection and incomplete immune reconstitution because of delayed HSC engraftment has restricted the use of cord blood in stem cell transplantation. In addition, UCB transplantation is associated with lack of transferred adoptive immunity, resulting in transplant related morbidity and mortality due to infectious complications. While the human neonatal immune system is considered to be immature compared with its adult counterpart, the role of the innate immune cells in the immunological naïvety is not fully understood.

Here, we describe the collection and characterisation of UCB units collected from a series of Irish mothers who delivered term infants by elective caesarean at a major Irish maternity hospital. Pre analytical variables included maternal factors such as gestation at delivery, blood group, age, medical history and BMI, and neonatal factors such as birth weight and gender. Mononuclear cells (MNC) were isolated and the immunological profile was analyzed by flow cytometry for the surface markers CD3, CD4, CD5, CD8, CD19, CD34, CD45, CD56, CD161 and the Vα24Jα17, Vα7.2, Vδ1, Vδ2 and Vδ3 T cell receptors. Progenitor cell colony forming ability was assessed by CFU-GM and BFU-E assays. Stimulation of T cells, by cross-linking of CD3 and CD28 using tetrameric antibodies, and Natural Killer (NK) cells, with IL12/IL18 and target cells, was assessed by measurement of surface expression of CD107a and intracellular production of IFNγ, IL4 and IL17a.

We showed no difference in the overall numbers of B cells, T cells (CD4⁺ and CD8⁺) and γδ T cells between UCB (n=23) and adult blood (n=4); however, NK cells (p<0.001) and mucosal-associated invariant T (MAIT) cells (p<0.0001) were significantly depleted in UCB. Stimulated UCB T cells and NK cells (n=17) showed significantly reduced IFNγ production compared to adult blood counterparts (n=4) (p<0.01) Following thaw of cryopreserved UCB MNC, we observed no significant change in CFU-GM, CD34⁺ cell concentration or immunological profile/function compared to fresh MNC (n=7).

Our study suggests that reduced numbers of circulating innate immune cell populations, as well as reduced cytokine responses following stimulation, may contribute to the naïve immunological phenotype and hyporesponsiveness of UCB. Further investigation and manipulation of these innate immune cell populations could improve UCB transferred adoptive immunity potential and assist in the management of post allogeneic stem cell transplant viral infections.