Regulation of Hematopoiesis In Vitro and In Vivo by Invariant NKT Cells.

Invariant NKT cells (iNKT cells) are a small subset of immunoregulatory T cells highly conserved in humans and mice. Upon activation by glycolipids presented by the MHC-like molecule CD1d, iNKT cells promptly secrete Th1/2 cytokines but also cytokines with hematopoietic potential such as IL-3 and GM-CSF. In mice, NKT cells activated by alpha-galactosylceramide (alphaGC), a potent glycolipid ligand, cause an in increase in extramedullary hematopoietic committed progenitor activity through secretion of these cytokines.

We tested the role of iNKT cells in regulating hematopoiesis under conditions of activation and in steady state hematopoiesis. We found that GM-CSF-secreting alphaGC -activated iNKT cells enhanced (by 64%, n=5, p<0.05) the myeloid clonogenic potential of human cord blood hematopoietic progenitors; conversely, in the absence of NKT cells short- and long-term progenitor activity is decreased: 48% reduction in GFU-GM frequency, n=6; p<0.05 and a reduction of 18–60% in LTC-IC frequency in 4 independent experiments. These findings suggest that NKT cells are implicated in the regulation of hematopoiesis both in the presence and absence of immune activation. In accordance with these findings, iNKT cell-deficient mice compared to wild type animals display impaired hematopoiesis characterized by peripheral blood cytopenia (43% and 32% reduction in leucocyte and platelet counts respectively, n=10, p<0.001), reduced marrow cellularity (reduced by 35%, n=13, p<0.001), lower frequency and absolute numbers per hind limb of hematopoietic stem cells (HSC) as assessed by the cKit+Lin-Sca-1+ phenotype (reduced by 40% n=8, p<0.05, and 61% n=8, p<0.001, respectively), and reduced early (50% and 68% reduction in frequency and absolute numbers of LTC-IC respectively, n=4, p<0.05) and late (40% and 57% reduction in frequency and total number of CFU-GM per hind limb respectively, n=8, p<0.05) hematopoietic progenitors. We also show that CD1d is expressed on human HSC and CD1d-expressing HSC display short- and long-term clonogenic potential and can present the glycolipid a-galactosylceramide to iNKT cells suggesting that the iNKT cell-mediated regulation of hematopoiesis might operate through cell contact of NKT cells with HSC and interaction of the invariant TCR with CD1d on HSC.

In conclusion, iNKT cells when activated enhance the clonogenic capacity of myeloid progenitors and can thus modulate innate immune responses. Additionally, they are required for maintenance of normal hematopoiesis in the absence of immune activation. Our findings offer further evidence for a concerted regulation of the immune and hematopoietic systems and the potential for new therapeutic approaches for the manipulation of hematopoiesis.