Involvement of T-Cells in the Bone Pathology of Multiple Myeloma

Introduction: Multiple myeloma (MM) is a plasma cell dyscrasia that is heavily dependent on a permissive BM micro-environment. Components of the WNT pathway have been implicated in this process and specifically, WNT-inhibitors such as Dkk1, have been blamed for the osteolytic lesions of MM. WNT signaling is required for proper osteoblast development while the pathway is also involved in enhancing the self renewal potential of various stem cells. A source of WNT ligands in the BM are the T-cells; when stimulated by parathormone, T-cells secrete wnt10b that affects hematopoietic stem cell numbers. In the present study, we evaluated WNT signaling in the T-cells of patients with MM in order to test whether these bystander T-cells have an altered phenotype that could influence disease pathology.

Methods: A total of 18 MM patients was analyzed after informed consent was obtained; the patients underwent BM biopsy for diagnostic and staging purposes. T-cells were isolated from BM mononuclear cells using magnetic beads with a purity of around 90%. Normal controls were T-cells isolated from healthy subjects. RNA was isolated and analyzed by RT-PCR for expression of PTH receptor (PTH1R), wnt10b and the canonical WNT signal transducer, b-catenin (bCAT).

Results and discussion: BM concentration of T-cells was 15%, close to the normal values, indicating no major deviation from normal conditions. Activity of the WNT pathway was estimated by bCAT mRNA levels; relative to the control, bCAT levels in MM T-cells were uniformily suppressed. In a total of 8 evaluable samples, there was a 3x decrease in bCAT levels. Although WNT activity could be influenced by different factors, there is a possibility that it may be related to PTH activity and PTH receptor expression on patient T-cells. In all our cases, calcium was recorded within normal range and PTH values were also normal. However, PTH1R mRNA levels in MM T-cells were 14.2x lower compared to normal T-cells, a value that argues for a biological role in disease pathogenesis. A direct consequence of low PTH1R mRNA levels, according to our hypothesis, would lead to a decrease in WNT10b. Indeed, WNT10b levels were on average 90% of the normal with relative wide variation; in 3 samples, there was an average 3.2x increase while in 8, the levels were on average 21% of normal. On the clinical level, lower wnt10b activity, was more often associated with bone pathology while higher wnt10b activity was more often associated with extramedullary plasmacytomas. Our data show that T-cells may not be bystanders in the MM disease progression and certain manifestations sush as bone disease, could be associated with, aberrant T cell function. Ongoing studies are under way to link clinical and laboratory data.