Myeloid-derived suppressor cells (MDSCs

Using transgenic mice that express a constitutively active version of STAT5b, we demonstrate that STAT5 plays a key role in governing B cell development and T cell homeostasis. STAT5 activation leads to a 10-fold increase in pro-B, but not pro-T, cells. Conversely, STAT5 signaling promotes the expansion of mature αβ T cells (6-fold increase) and γδ and NK T cells (3- to 4-fold increase), but not of mature B cells. In addition, STAT5 activation has dramatically divergent effects on CD8+ vs CD4+ T cells, leading to the selective expansion of CD8+ memory-like T cells and CD4+CD25+ regulatory T cells. These results establish that activation of STAT5 is the primary mechanism underlying both IL-7/IL-15-dependent homeostatic proliferation of naive and memory CD8+ T cells and IL-2-dependent development of CD4+CD25+ regulatory T cells.

Lipid-rich atherosclerotic plaques are vulnerable, and their rupture can cause the formation of a platelet- and fibrin-rich thrombus leading to myocardial infarction and ischemic stroke. Although the role of plaque-based tissue factor as stimulator of blood coagulation has been recognized, it is not known whether plaques can cause thrombus formation through direct activation of platelets. We isolated lipid-rich atheromatous plaques from 60 patients with carotid stenosis and identified morphologically diverse collagen type I- and type III-positive structures in the plaques that directly stimulated adhesion, dense granule secretion, and aggregation of platelets in buffer, plasma, and blood. This material also elicited platelet-monocyte aggregation and platelet-dependent blood coagulation. Plaques exposed to flowing blood at arterial wall shear rate induced platelets to adhere to and spread on the collagenous structures, triggering subsequent thrombus formation. Plaque-induced platelet thrombus formation was observed in fully anticoagulated blood (i.e., in the absence of tissue factor-mediated coagulation). Mice platelets lacking glycoprotein VI (GPVI) were unable to adhere to atheromatous plaque or form thrombi. Human platelet thrombus formation onto plaques in flowing blood was completely blocked by GPVI inhibition with the antibody 10B12 but not affected by integrin alpha2beta1 inhibition with 6F1 mAb. Moreover, the initial platelet response, shape change, induced by plaque was blocked by GPVI inhibition but not with alpha2beta1 antagonists (6F1 mAb or GFOGER-GPP peptide). Pretreatment of plaques with collagenase or anti-collagen type I and anti-collagen type III antibodies abolished plaque-induced platelet activation. Our results indicate that morphologically diverse collagen type I- and collagen type III-containing structures in lipid-rich atherosclerotic plaques stimulate thrombus formation by activating platelet GPVI. This platelet collagen receptor, essential for plaque-induced thrombus formation, presents a promising new anti-thrombotic target for the prevention of ischemic cardiovascular diseases.

Signals initiated by the IL7R are required for B cell development. However, the roles that distinct IL7R-induced signaling pathways play in this process remains unclear. To identify the function of the Raf and STAT5 pathways in IL7R-dependent B cell development, we used transgenic mice that express constitutively active forms of Raf (Raf-CAAX) or STAT5 (STAT5b-CA) throughout lymphocyte development. Both Raf-CAAX and STAT5b-CA mice exhibit large increases in pro-B cells. However, crossing the Raf-CAAX transgene onto the IL7R−/− background fails to rescue B cell development. In contrast, STAT5 activation selectively restores B cell expansion in IL7R−/− mice. Notably, the expansion of pro-B cells in STAT5b-CA mice correlated with an increase in cyclin D2, pim-1, and bcl-xL expression, suggesting that STAT5 directly affects pro-B cell proliferation and survival. In addition, STAT5 activation also restored B cell differentiation in IL7R−/− mice as determined by 1) the restoration of VH Ig gene rearrangement and 2) the appearance of immature and mature B cell subsets. These findings establish STAT5 as the key player entraining B cell development downstream of the IL7R.

The molecular mechanisms regulating lymphocyte lineage commitment remain poorly characterized. To explore the role of the IL7R in this process, we generated transgenic mice that express a constitutively active form of STAT5 (STAT5b-CA), a key downstream IL7R effector, throughout lymphocyte development. STAT5b-CA mice exhibit a 40-fold increase in pro-B cells in the thymus. As documented by BrdU labeling studies, this increase is not due to enhanced B cell proliferation. Thymic pro-B cells in STAT5b-CA mice show a modest increase in cell survival (∼4-fold), which correlates with bcl-xL expression. However, bcl-xL transgenic mice do not show increases in thymic B cell numbers. Thus, STAT5-dependent bcl-xL up-regulation and enhanced B cell survival are not sufficient to drive the thymic B cell development observed in STAT5b-CA mice. Importantly, thymic pro-B cells in STAT5b-CA mice are derived from early T cell progenitors (ETPs), suggesting that STAT5 acts by altering ETP lineage commitment. Supporting this hypothesis, STAT5 binds to the pax5 promoter in ETPs from STAT5b-CA mice and induces pax5, a master regulator of B cell development. Conversely, STAT5b-CA mice exhibit a decrease in the DN1b subset of ETPs, demonstrating that STAT5 activation inhibits early T cell differentiation or lineage commitment. On the basis of these findings, we propose that the observed expression of the IL-7R on common lymphoid progenitors, but not ETPs, results in differential STAT5 signaling within these distinct progenitor populations and thus helps ensure appropriate development of B cells and T cells in the bone marrow and thymic environments, respectively.