Insights Into the Differential Induction of PRDM1 in CLL Cells: Relevance for Pathobiology and Disease Therapeutics

Abstract 1763

Chronic Lymphocytic Leukaemia (CLL) is characterised by a clonal expansion of activated CD5+ mature B-lymphocytes. On antigen recognition by the B-cell receptor (BCR), a normal B cell may undergo apoptosis, progress to late B-cell differentiation, or become anergic. While many stimuli have been implicated in their survival and proliferation, CLL cells are selected and likely continually stimulated by antigen. Antigen recognition does not appear to preferentially induce either plasma-cell (PC) differentiation or apoptosis, but instead CLL lymphocytes remain activated and proliferate in an anergy-like state. The inability of CLL cells to differentiate into PC's, produce antibody and potentially eliminate the stimulating antigen is not well understood. PRDM1, a tumour suppressor gene, is essential for PC differentiation and antibody production. Its loss of expression, and function, has recently been shown to cause the manifestation of an activated B cell (ABC)-like Diffuse Large B cell Lymphoma (DLBCL). Similar to ABC-DLBCL, CLL cells also exhibit constitutive activation of the canonical NF-κB pathway. Whilst it is known that CLL cells express little or no PRDM1, the effect of PC inducing stimuli on PRDM1 expression in CLL cells has not been reported. The aim of this study was therefore to establish whether functional PRDM1 could be induced in CLL.

We first studied the effects of IL-21 and CpG-ODN on CLL cells because of their potent effect on inducing PRDM1 in normal B cells. We found that after in vitro stimulation with either IL-21 or CpG-ODN, PRDM1 could be readily induced in the CLL cells from ∼50% of patients. Induction of PRDM1 was associated with PC differentiation and antibody secretion from these responsive clones (R-CLL). In contrast, a failure to induce PRDM1 from non-responsive CLL lymphocytes (NR-CLL) following stimulation was associated with a lack of PC differentiation and antibody production. Failure to induce PRDM1 expression in NR-CLL was due to a block in transcription despite efficient and preserved signal transduction from the IL-21 receptor or toll-like receptor 9 (TLR9; CpG-ODN receptor) as evidenced by induction of phosphorylated Tyr₇₀₅ in STAT3 and activation of the p65 subunit of NF-κB respectively. Furthermore, there was no difference in induction of Mcl-1 (IL-21R target) or IκBα and IκBζ (TLR9 targets) in R- and NR-CLL clones after treatment with IL-21 or CpG-ODN. Importantly, additional co-stimuli such as CD40L and BCR crosslinking failed to overcome the block in PRDM1 transcription in NR-CLL. The failure to induce PRDM1 in NR-CLL was associated, significantly, with IGVH1-69 gene usage (n=8/8; P=0.03), while the IGVH3 gene-family expressing clones showed a tendency to induce PRDM1 (18/29; P=0.065). There was no significant association of PRDM1 induction with other well-characterised prognostic variables in CLL.

We conclude and hypothesise that a failure to induce PRDM1 expression may be of pathobiological and therapeutic significance in some CLL patients (NR-CLL). A correlation with particular IGVH gene usage suggests a link to the antigen that might have selected and is driving the disease in vivo. Furthermore, this work illustrates that strategies that promote a forced induction of PRDM1 (e.g. with either IL-21 or CpG-ODN), may be exploited for therapeutic benefit in a subset of CLL patients, to induce terminal differentiation and cell death.