Sheinei J. Saleem

A brief overview of mouse models of pulmonary arterial hypertension: problems and prospects

Many chronic pulmonary diseases are associated with pulmonary hypertension (PH) and pulmonary vascular remodeling, which is a term that continues to be used to describe a wide spectrum of vascular abnormalities. Pulmonary vascular structural changes frequently increase pulmonary vascular resistance, causing PH and right heart failure. Although rat models had been standard models of PH research, in more recent years the availability of genetically engineered mice has made this species attractive for many investigators. Here we review a large amount of data derived from experimental PH reports published since 1996. These studies using wild-type and genetically designed mice illustrate the challenges and opportunities provided by these models. Hemodynamic measurements are difficult to obtain in mice, and right heart failure has not been investigated in mice. Anatomical, cellular, and genetic differences distinguish mice and rats, and pharmacogenomics may explain the degree of PH and the particular mode of pulmonary vascular adaptation and also the response of the right ventricle.

Cutting Edge: Mast Cells Critically Augment Myeloid-Derived Suppressor Cell Activity

Myeloid-derived suppressor cells (MDSCs) are primarily recognized for their immunosuppressive properties in malignant disease. However, their interaction with other innate immune cells and their regulation of immune responses, such as in parasitic infection, necessitate further characterization. We used our previously published mouse model of MDSC accumulation to examine the immunoregulatory role of MDSCs in B16 melanoma metastasis and Nippostrongylus brasiliensis infection. In this study, we demonstrate that the activity of MDSCs is dependent on the immune stimuli and subset induced. Monocytic MDSCs predictably suppressed antitumor immune responses but granulocytic MDSCs surprisingly enhanced the clearance of N. brasiliensis infection. Intriguingly, both results were dependent on MDSC interaction with mast cells (MCs), as demonstrated by adoptive-transfer studies in MC-deficient (KitWsh/Wsh) mice. These findings were further supported by ex vivo cocultures of MCs and MDSCs, indicating a synergistic increase in cytokine production. Thus, MCs can enhance both immunosuppressive and immunosupportive functions of MDSCs.

ADAM10 Overexpression Shifts Lympho- and Myelopoiesis by Dysregulating Site 2/Site 3 Cleavage Products of Notch

Although the physiological consequences of Notch signaling in hematopoiesis have been extensively studied, the differential effects of individual notch cleavage products remain to be elucidated. Given that ADAM10 is a critical regulator of Notch and that its deletion is embryonically lethal, we generated mice that overexpress ADAM10 (ADAM10 transgenic [A10Tg]) at early stages of lympho- and myeloid development. Transgene expression resulted in abrogated B cell development, delayed T cell development in the thymus, and unexpected systemic expansion of CD11b+Gr-1+ cells, also known as myeloid-derived suppressor cells. Mixed bone marrow reconstitution assays demonstrated that transgene expression altered hematopoiesis via a cell-intrinsic mechanism. Consistent with previously reported observations, we hypothesized that ADAM10 overexpression dysregulated Notch by uncoupling the highly regulated proteolysis of Notch receptors. This was confirmed using an in vitro model of hematopoiesis via culturing A10Tg hematopoietic Lineage−Sca-1+c-Kit+ cells with OP-9 stromal cells in the presence or absence of Delta-like 1, a primary ligand for Notch. Blockade of the site 2 (S2) and site 3 (S3) cleavage of the Notch receptor demonstrated differential effects on hematopoiesis. OP9-DL1 cultures containing the ADAM10 inhibitor (S2 cleavage site) enhanced and rescued B cell development from wild-type and A10Tg Lineage−Sca-1+c-Kit+ cells, respectively. In contrast, blockade of γ-secretase at the S3 cleavage site induced accumulation of the S2 product and consequently prevented B cell development and resulted in myeloid cell accumulation. Collectively, these findings indicate that the differential cleavage of Notch into S2 and S3 products regulated by ADAM10 is critical to hematopoietic cell-fate determination.

Mast cell histamine promotes the immunoregulatory activity of myeloid-derived suppressor cells

It has been shown recently that MCs are required for differential regulation of the immune response by granulocytic versus monocytic MDSCs. Granulocytic MDSCs promoted parasite clearance, whereas monocytic MDSCs enhanced tumor progression; both activities were abrogated in MC‐deficient mice. Herein, we demonstrate that the lack of MCs also influences MDSC trafficking. Preferential trafficking to the liver was not seen in MC‐deficient mice. In addition, evidence that the MC mediator histamine was important in MDSC trafficking and activation is also shown. MDSCs express HR1–3. Blockade of these receptors by HR1 or HR2 antagonists reversed the histamine enhancement of MDSC survival and proliferation observed in cell culture. In addition, histamine differentially influenced Arg1 and iNOS gene expression in MDSCs and greatly enhanced IL‐4 and IL‐13 message, especially in granulocytic MDSCs. Evidence that histamine influenced activity seen in vitro translated to in vivo when HR1 and HR2 antagonists blocked the effect of MDSCs on parasite expulsion and tumor metastasis. All of these data support the MDSC‐mediated promotion of Th2 immunity, leading to the suggestion that allergic‐prone individuals would have elevated MDSC levels. This was directly demonstrated by looking at the relative MDSC levels in allergic versus control patients. Monocytic MDSCs trended higher, whereas granulocytic MDSCs were increased significantly in allergic patients. Taken together, our studies indicate that MCs and MC‐released histamine are critical for MDSC‐mediated immune regulation, and this interaction should be taken into consideration for therapeutic interventions that target MDSCs.

The emergence of ADAM10 as a regulator of lymphocyte development and autoimmunity

Proteolytic processing of transmembrane receptors and ligands can have a dramatic impact on cell signaling processes and subsequent cellular responses, including activation and differentiation. A member of the disintegrin and metalloproteinase family, ADAM10, has emerged as a prominent regulator of numerous receptors and ligands, including Notch and CD23. Here, we review studies resulting from the recent generation of ADAM10 conditional knockout mice which revealed a critical role for ADAM10 in Notch-dependent lymphocyte development. Additionally, we discuss results of numerous in vitro and ex vivo studies indicating that ADAM10 regulates the production of multiple secreted factors that contribute to autoimmune reactions.

Myeloid-derived suppressor cells enhance IgE-mediated mast cell responses

Mast cells and MDSCs are increased by parasitic infection and tumor growth. We previously demonstrated that enhanced MDSC development in ADAM10 transgenic mice yielded resistance to Nb infection and that coculturing MDSCs and mast cells enhanced cytokine production. In the current work, we show that MDSC‐mast cell coculture selectively enhances IgE‐mediated cytokine secretion among mast cells, without increasing MDSC cytokine production. This effect was independent of cell contact and elicited by Ly6C+ and Ly6C/G+ MDSC subsets. These interactions were functionally important. MDSC depletion with the FDA‐approved drug gemcitabine exacerbated Nb or Trichinella spiralis infection and reduced mast cell‐dependent AHR and lung inflammation. Adoptive transfer of MDSC worsened AHR in WT but not mast cell‐deficient Wsh/Wsh mice. These data support the hypothesis that MDSCs enhance mast cell inflammatory responses and demonstrate that this interaction can be altered by an existing chemotherapeutic.