SIRPα controls CD47-dependent platelet clearance in mice and humans

Over the last decade, more data has revealed that increased surface expression of the "don't eat me" CD47 protein on cancer cells plays a role in immune evasion and tumor progression, with CD47 blockade emerging as a new therapy in immuno-oncology. CD47 is critical in regulating cell homeostasis and clearance, as binding of CD47 to the inhibitory receptor SIRPα can prevent phagocytosis and macrophage-mediated cell clearance. The purpose of this study was to examine the role of the CD47-SIRPα signal in platelet homeostasis and clearance. Therapeutic reagents targeting the CD47-SIRPα axis are very promising for treatment of hematologic malignancies and solid tumors, but lead to transient anemia or thrombocytopenia in a subset of patients. We found that platelet homeostatic clearance is regulated through the CD47-SIRPα axis and that therapeutic blockade to disrupt this interaction in mice and in humans has a significant impact on platelet levels. Furthermore, we identified genetic variations at the SIRPA locus that impact platelet levels in humans such that higher SIRPA gene expression is associated with higher platelet levels. SIRPA expression at either end of the normal range may affect clinical outcomes of treatment with anti-CD47 therapy.

Novel Association of Lyme disease, Age, and Atopic Dermatitis

Borrelia burgdorferi is a bacterial spirochete that can cause Lyme disease (LD) after infecting a susceptible host. Immune responses to the bacteria are highly variable and host specific. The murine substrain, C3H/HeJ, is a frequently utilized model for LD. Interestingly, over a prolonged infection, mice develop dermatitis on tail skin, which shares critical features with human skin. Female C3H/HeJ mice aged 5–8 weeks, 1 year, or 2 years were infected intraperitoneally with 105 B. burgdorferi. Dermatitis was evaluated by gross examination and histology. Dermatitis worsened over the course of untreated infection, with ulceration, hemorrhaging, flaking, hair loss, and dark lesions as well as spongiosis and acanthosis. These features of dermatitis were present in infected mice after 1 year of age. This relationship among LD, atopic dermatitis, and host age seen in the C3H/HeJ mouse model is consistent with a large pool (342,499) of human epidemiological data from Finland. We identified 5,248 individuals with LD and 17,233 with atopic dermatitis in FinnGen. Retrospective analysis shows LD is associated with atopic dermatitis (OR = 1.91 [1.68 −2.37], P < 2e−16). More visits due to LD complications (3 or more visits versus 1 visit) were associated with atopic dermatitis (OR = 2.19 [1.35–3.55], P = 0.0014) and risk of developing atopic dermatitis over time (HR = 2.26 [1.54–3.95], P = 0.0017). Data from mice and humans reveal a novel relationship among LD, age, and atopic dermatitis. Through defined pathological scoring, we demonstrate the onset of murine atopic dermatitis with B. burgdorferi infection, which is further exacerbated by host age at time of infection, and likewise report a similar association in human epidemiological data from FinnGen.

Research was supported by the Fairbairn Family foundation; Bay Area Lyme Foundation; the Younger family foundation; the Robert J. Kleberg, Jr., and Helen C. Kleberg Foundation; the Virginia and D. K. Ludwig Fund for Cancer Research; AML grant R01CA086017; the PCBC from NIHLB U01HL099999; as well as grant U19AI109662. M.C.T. was supported by Stanford Immunology training grant 5T32AI007290, and the NIH NRSA 1 F32 AI124558-01 award. L.B.T.D. was supported by a Stanford Diversifying Academia Recruiting Excellence fellowship. S.G. was supported by the California Institute for Regenerative Medicine. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

CD47 Blockade Leads to Chemokine-Dependent Monocyte Infiltration and Loss of B Cells from the Splenic Marginal Zone

CD47 is an important innate immune checkpoint through its interaction with its inhibitory receptor on macrophages, signal-regulatory protein α (SIRPα). Therapeutic blockade of CD47-SIRPα interactions is a promising immuno-oncology treatment that promotes clearance of cancer cells. However, CD47-SIRPα interactions also maintain homeostatic lymphocyte levels. In this study, we report that the mouse splenic marginal zone B cell population is dependent on intact CD47-SIRPα interactions and blockade of CD47 leads to the loss of these cells. This depletion is accompanied by elevated levels of monocyte-recruiting chemokines CCL2 and CCL7 and infiltration of CCR2⁺Ly6C^hi monocytes into the mouse spleen. In the absence of CCR2 signaling, there is no infiltration and reduced marginal zone B cell depletion. These data suggest that CD47 blockade leads to clearance of splenic marginal zone B cells.

Pathology updates and diagnostic approaches to hemophagocytic lymphohistiocytosis

Hemophagocytic lymphohistiocytosis (HLH) is a complex, often under-recognized hyperinflammatory immune dysregulation syndrome arising in a diverse range of clinical scenarios and conditions. The accurate and timely diagnosis of HLH is crucial for patient survival, and usually requires a high level of clinical suspicion. The histologic corollary to clinical HLH - hemophagocytosis - is neither necessary nor sufficient for the diagnosis of HLH, as it may be seen in a variety of reactive conditions and may be absent in true HLH. Nevertheless, the finding of hemophagocytosis in specific clinical situations should prompt consideration of HLH and further testing to exclude the condition. While traditionally described in bone marrow, identification of hemophagocytosis in other tissues, including lymphoid, splenic, liver, or neural tissue, can be an important asset to the overall recognition of HLH. In this review we discuss the underlying pathophysiology and etiologies of HLH, morphologic aspects of hemophagocytosis and its associated histologic findings in different tissues, and give a brief overview of diagnostic criteria and clinical evaluation.

Enasidenib drives human erythroid differentiation independently of isocitrate dehydrogenase 2

Cancer-related anemia is present in more than 60% of newly diagnosed cancer patients and is associated with substantial morbidity and high medical costs. Drugs that enhance erythropoiesis are urgently required to decrease transfusion rates and improve quality of life. Clinical studies have observed an unexpected improvement in hemoglobin and RBC transfusion-independence in patients with acute myeloid leukemia (AML) treated with the isocitrate dehydrogenase 2 (IDH2) mutant-specific inhibitor enasidenib, leading to improved quality of life without a reduction in AML disease burden. Here, we demonstrate that enasidenib enhanced human erythroid differentiation of hematopoietic progenitors. The phenomenon was not observed with other IDH1/2 inhibitors and occurred in IDH2-deficient CRISPR-engineered progenitors independently of D-2-hydroxyglutarate. The effect of enasidenib on hematopoietic progenitors was mediated by protoporphyrin accumulation, driving heme production and erythroid differentiation in committed CD71+ progenitors rather than hematopoietic stem cells. Our results position enasidenib as a promising therapeutic agent for improvement of anemia and provide the basis for a clinical trial using enasidenib to decrease transfusion dependence in a wide array of clinical contexts.