Regulation of bone homeostasis by MERTK and TYRO3

The fine equilibrium of bone homeostasis is maintained by bone-forming osteoblasts and bone-resorbing osteoclasts. Here, we show that TAM receptors MERTK and TYRO3 exert reciprocal effects in osteoblast biology: Osteoblast-targeted deletion of MERTK promotes increased bone mass in healthy mice and mice with cancer-induced bone loss, whereas knockout of TYRO3 in osteoblasts shows the opposite phenotype. Functionally, the interaction of MERTK with its ligand PROS1 negatively regulates osteoblast differentiation via inducing the VAV2-RHOA-ROCK axis leading to increased cell contractility and motility while TYRO3 antagonizes this effect. Consequently, pharmacologic MERTK blockade by the small molecule inhibitor R992 increases osteoblast numbers and bone formation in mice. Furthermore, R992 counteracts cancer-induced bone loss, reduces bone metastasis and prolongs survival in preclinical models of multiple myeloma, breast- and lung cancer. In summary, MERTK and TYRO3 represent potent regulators of bone homeostasis with cell-type specific functions and MERTK blockade represents an osteoanabolic therapy with implications in cancer and beyond.

MerTK Cleavage on Resident Cardiac Macrophages Compromises Repair After Myocardial Ischemia Reperfusion Injury

RATIONALE

Clinical benefits of reperfusion after myocardial infarction are offset by maladaptive innate immune cell function, and therapeutic interventions are lacking.

OBJECTIVE

We sought to test the significance of phagocytic clearance by resident and recruited phagocytes after myocardial ischemia reperfusion.

METHODS AND RESULTS

In humans, we discovered that clinical reperfusion after myocardial infarction led to significant elevation of the soluble form of MerTK (myeloid-epithelial-reproductive tyrosine kinase; ie, soluble MER), a critical biomarker of compromised phagocytosis by innate macrophages. In reperfused mice, macrophage Mertk deficiency led to decreased cardiac wound debridement, increased infarct size, and depressed cardiac function, newly implicating MerTK in cardiac repair after myocardial ischemia reperfusion. More notably, Mertk(CR) mice, which are resistant to cleavage, showed significantly reduced infarct sizes and improved systolic function. In contrast to other cardiac phagocyte subsets, resident cardiac MHCII^LOCCR2^- (major histocompatibility complex II/C-C motif chemokine receptor type 2) macrophages expressed higher levels of MerTK and, when exposed to apoptotic cells, secreted proreparative cytokines, including transforming growth factor-β. Mertk deficiency compromised the accumulation of MHCII^LO phagocytes, and this was rescued in Mertk(CR) mice. Interestingly, blockade of CCR2-dependent monocyte infiltration into the heart reduced soluble MER levels post-ischemia reperfusion.

CONCLUSIONS

Our data implicate monocyte-induced MerTK cleavage on proreparative MHCII^LO cardiac macrophages as a novel contributor and therapeutic target of reperfusion injury.

Regulation of chronic lung inflammation to house dust mite allergen by Wnt antagonist (CAM1P.144)

Recruitment of leukocytes in response to allergen is an important step in the initiation of airway inflammation. Here we demonstrate that the Wnt antagonist Dkk-1 regulates the formation of leukocyte-platelet aggregates (LPAs) following house dust mite (HDM) allergen challenge. We show that circulating Dkk-1 is elevated after recurrent HDM allergen challenges and the source is activated platelets. The increase of P-selectin glycoprotein ligand-1 (PSGL-1) expression in leukocytes was mediated by Dkk-1 while other cell adhesion molecules such as ICAM-1 or LFA-1 were only marginally affected. The formation of LPAs was markedly reduced in Dkk-1 hypomorphic doubleridge (Dkk-1d/d) mice that had a 90% reduction of circulating Dkk-1. Recurrent allergen challenges in Dkk-1d/d mice showed significantly reduced infiltration of leukocytes including neutrophils in the lung compared to wildtype mice. Importantly, we show that type 2 cytokines (e.g. IL-4, IL-5 and IL-13) from CD4 T cells in mediastinal lymph nodes (med LNs) of Dkk-1d/d mice were substantially reduced compared to wildtype mice following allergen exposure. Consistently, we observed Gata-3 and c-Maf induction in Th2 cells did not occur in med LN CD4 T cells from Dkk-1d/d mice. Therefore, our results demonstrate that the Wnt antagonist from activated platelets regulates the early stage of leukocyte infiltration in the challenged tissue, thereby regulating inflammation and type 2 immune responses.

Role of the TAM receptor Axl in differentiation and activation of B cells (P4058)

In dendritic cells (DCs), TLR activation upregulates TAM receptor expression and these TAM receptors initiated pathways subsequently shut down cytokine production. Moreover, TAM receptor deficient mice develop a severe SLE-like disease. TLR9 and TLR7 play a key role in the detection of autoantigens by both DCs and B cells in the context of systemic autoimmunity. To better understand the direct effects of TAM receptors on B cells, we have used BCR Tg mice (AM14) that recognize autologous IgG2a. AM14 B cells are activated by immune complexes (ICs) that incorporate either DNA or RNA through pathways dependent on TLR9 and TLR7, respectively. Contrary to DCs, expression of the TAM receptor Axl is not induced by TLR ligands but is dependent on crosslinking of the BCR per se. However, BCR/TLR9, but not BCR/TLR7, co-engagement inhibits Axl upregulation, pointing to a role for TLR9 in the regulation of Axl expression. Activation of AM14 B cells by RNA-containing ICs induces TLR7-dependent differentiation into plasma cells and these plasma cells express high levels of Axl. In contrast, BCR/TLR9-dependent activation of AM14 B cells by DNA-containing ICs blocks Axl expression and does not lead to plasma cell differentiation. These data identify unique outcomes of BCR/TLR7 and BCR/TLR9 co-engagement on B cell differentiation and Axl expression. Furthermore, these B cell intrinsic responses may account for the opposing roles of TLR7 and TLR9 in the regulation of systemic autoimmunity.

Stromal-cell regulation of natural killer cell differentiation

Natural killer (NK) cells are bone-marrow-derived lymphocytes that play a crucial role in host defense against some viral and bacterial infections, as well as against tumors. Their phenotypic and functional maturation requires intimate interactions between the bone marrow stroma and committed precursors. In parallel to the identification of several phenotypic and functional stages of NK cell development, recent studies have shed new light on the role of stromal cells in driving functional maturation of NK cells. In this review, we provide an overview of the role of bone marrow microenvironment in NK cell differentiation.