T Cell Mediated Suppression of Hematopoiesis in Myeloproliferative Neoplasm

Background:

Increased inflammation is an established feature of MPN, but its precise role in disease pathogenesis has yet to be clearly defined. T cell mediated insult on hematopoietic stem cells can drive expansion of clones that have mutated in such a way as to escape these suppressive and/or apoptotic cues (this mechanism is clearly illustrated in aplastic anemia). To investigate whether T cell mediated suppression of hematopoiesis occurs in MPN we interrogated the T lymphocyte compartment of MPN patients, guided by abnormalities known to be present in aplastic anemia.

Methods and Results:

We first asked how selective depletion of T cells from peripheral blood mononuclear cells (MNC) would affect myeloid colony formation in MPN as compared to normal controls. MNC were depleted of T cells using CD3 magnetic beads or mock depleted and plated side by side in methylcellulose (+SCF, IL-3, Epo). Removal of CD3⁺ T cells from bulk MNC reduced colony formation in all 29 normal control samples tested, consistent with the ability of normal lymphocytes to enhance colony formation. In contrast, removal of CD3⁺ T cells from MPN MNC resulted in an increase in colony formation in 20 out of 51 MPN patient samples (containing an equal distribution of polycythemia vera, essential thrombocythemia, and myelofibrosis). From JAK2^V617F-positive MPN patients, colonies were plucked and genotyped for JAK2^V617F mutational status. The fraction of JAK2^WT colonies increased with removal of T cells, demonstrating that JAK2^WT progenitors are more sensitive to T cell mediated suppression as compared to their JAK2^V617F counterparts. The observation that removal of T cells enhances myeloid colony forming ability of some MPN patients suggests that T mediated suppression of hematopoiesis may be occurring in MPN.

Next, we tested whether MPN T cells have the ability to suppress colony formation of normal hematopoietic progenitors. CD3⁺ T cells from MPN (n=2) and normal controls (n=2) were added to normal control mobilized peripheral blood CD34⁺ cells (n=3). Each CD34⁺ sample was plated in methylcellulose by itself, with the addition of 100,000 MPN T cells/1.1ml or with the addition of 100,000 normal control T cells/1.1ml. For each CD34⁺ sample a unique MPN patient and normal control was used as the source of T cells. Addition of MPN T cells reduced colony formation in one of three CD34⁺ samples, whereas normal control T cells enhanced colony formation of all three CD34⁺ samples. In the remaining two CD34⁺ samples MPN T cells had an attenuated ability to enhance colony formation as compared to the normal control T cells.

Conclusion:

T-cell mediated suppression of myelopoiesis in vitro is demonstrable in MPN and may be a contributor to the anemia associated with myelofibrosis. Taken together, these data provide the rationale for further investigation into T mediated suppression of hematopoietic progenitors in MPN patients.