This work provides novel insights into the intersection between two critical areas of immunology, the CD1d-invariant NKT (iNKT) axis and B cells. CD1d is a non-polymorphic, MHC class I-like molecule, which presents phospho- and glycosphingo-lipid antigens to a subset of CD1d-restricted T cells called iNKT cells. CD1d is expressed on a variety of antigen presenting cells and the CD1d-iNKT cell axis regulates nearly all aspects of the innate and adaptive immune response. Expression of CD1d on B cells allows these cells to form cognate interactions with iNKT cells. Emerging evidence suggests, however, that expression of CD1d on B cells is variable, both on “normal” B cells during humoral immune responses, and also on “pathological” B cells in certain B cell disorders.
In this work, I investigate in detail the expression of CD1d on B cells across a range of conditions. Using both human and murine germinal centre (GC) B cells as a model for normal B cells, I show for the first time that CD1d expression changes dynamically, both at the surface protein and transcriptional level. CD1d falls to a nadir as a naïve B cell enters the GC, and subsequently rises again in post GC B cells. I then provide evidence that the loss of CD1d expression is paralleled in pathological B cells, specifically in Epstein-Barr Virus infection of B cells and the plasma cell disorder Multiple Myeloma. In these conditions, CD1d is again downregulated at both the surface and transcriptional level.
Having established that CD1d expression is lost in certain scenarios, I attempt to elucidate the biological significance of this downregulation. I approach this aim by attempting to constitutively express CD1d in murine GC B cells, and subsequently assessing the GC reaction. Constitutive expression of CD1d is achieved by two distinct methods, either by using adoptive transfer of Cd1d1 transduced haematopoietic stem cells or by using a CD1d transgenic mouse model. I show that both of these approaches represent a feasible way to constitutively express CD1d in murine GC B cells, but cannot establish a definitive biological role for CD1d downregulation in murine GC B cells.
Finally, I investigate the transcriptional mechanisms governing the downregulation of CD1d, in either normal or pathological B cells. By analysing GC and MM B cells, I provide the first evidence that both retinoic acid signalling and bivalent chromatin domains act as a dual regulatory mechanism of CD1d. These findings demonstrate a new concept in the field of MM biology, specifically that MM B cells (like lymphomas) are able to “hijack” physiological pathways for disease propagation. Although previous work has highlighted the ability of all-trans retinoic acid (ATRA) alone to increase CD1d expression, this uncovering of a dual mechanism provides the rationale to use both ATRA and the polycomb-repressive complex inhibitor GSK343 to restore CD1d expression. I show that this is a more effective strategy than ATRA alone in increasing CD1d expression. Although this will need to be validated in further in vivo models, this ability to further increase CD1d expression may be of great therapeutic importance in the emerging field of tumour immunotherapy.Open Acces
