Immune Mechanisms in Inflammatory Anemia.

Maintaining the correct number of healthy red blood cells (RBCs) is critical for proper oxygenation of tissues throughout the body. Therefore, RBC homeostasis is a tightly controlled balance between RBC production and RBC clearance, through the processes of erythropoiesis and macrophage hemophagocytosis, respectively. However, during the inflammation associated with infectious, autoimmune, or inflammatory diseases this homeostatic process is often dysregulated, leading to acute or chronic anemia. In each disease setting, multiple mechanisms typically contribute to the development of inflammatory anemia, impinging on both sides of the RBC production and RBC clearance equation. These mechanisms include both direct and indirect effects of inflammatory cytokines and innate sensing. Here, we focus on common innate and adaptive immune mechanisms that contribute to inflammatory anemias using examples from several diseases, including hemophagocytic lymphohistiocytosis/macrophage activation syndrome, severe malarial anemia during Plasmodium infection, and systemic lupus erythematosus, among others.

Human ZBP1 induces cell death-independent inflammatory signaling via RIPK3 and RIPK1.

ZBP1 is an interferon-induced cytosolic nucleic acid sensor that facilitates antiviral responses via RIPK3. Although ZBP1-mediated programmed cell death is widely described, whether and how it promotes inflammatory signaling is unclear. Here, we report a ZBP1-induced inflammatory signaling pathway mediated by K63- and M1-linked ubiquitin chains, which depends on RIPK1 and RIPK3 as scaffolds independently of cell death. In human HT29 cells, ZBP1 associated with RIPK1 and RIPK3 as well as ubiquitin ligases cIAP1 and LUBAC. ZBP1-induced K63- and M1-linked ubiquitination of RIPK1 and ZBP1 to promote TAK1- and IKK-mediated inflammatory signaling and cytokine production. Inhibition of caspase activity suppressed ZBP1-induced cell death but enhanced cytokine production in a RIPK1- and RIPK3 kinase activity-dependent manner. Lastly, we provide evidence that ZBP1 signaling contributes to SARS-CoV-2-induced cytokine production. Taken together, we describe a ZBP1-RIPK3-RIPK1-mediated inflammatory signaling pathway relayed by the scaffolding role of RIPKs and regulated by caspases, which may induce inflammation when ZBP1 is activated below the threshold needed to trigger a cell death response.

Signals governing monocyte differentiation during inflammation.

Monocytes are innate immune cells that develop in the bone marrow and are continually released into circulation, where they are poised to enter tissues in response to homeostatic or inflammatory cues. Monocytes are highly plastic cells that can differentiate in tissues into a variety of monocyte-derived cells to replace resident tissue macrophages, promote inflammatory responses, or resolution of inflammation. As such, monocytes can support tissue homeostasis as well as productive and pathogenic immune responses. Recent work shows previously unappreciated heterogeneity in monocyte development and differentiation in the steady state and during infectious, autoimmune, and inflammatory diseases. Monocyte-derived cells can differentiate via signals from cytokines, pattern recognition receptors or other factors, which can influence development in the bone marrow or in tissues. An improved understanding of these monocyte-derived cells and the signals that drive their differentiation in distinct inflammatory settings could allow for targeting these pathways in pathological inflammation.

RIPK3 Activation Leads to Cytokine Synthesis that Continues after Loss of Cell Membrane Integrity.

Necroptosis is a form of programmed cell death that is defined by activation of the kinase RIPK3 and subsequent cell membrane permeabilization by the effector MLKL. RIPK3 activation can also promote immune responses via production of cytokines and chemokines. How active cytokine production is coordinated with the terminal process of necroptosis is unclear. Here, we report that cytokine production continues within necroptotic cells even after they have lost cell membrane integrity and irreversibly committed to death. This continued cytokine production is dependent on mRNA translation and requires maintenance of endoplasmic reticulum integrity that remains after plasma membrane integrity is lost. The continued translation of cytokines by cellular corpses contributes to necroptotic cell uptake by innate immune cells and priming of adaptive immune responses to antigens associated with necroptotic corpses. These findings imply that cell death and production of inflammatory mediators are coordinated to optimize the immunogenicity of necroptotic cells.

Intratumoral activation of the necroptotic pathway components RIPK1 and RIPK3 potentiates antitumor immunity.

Although the signaling events that induce different forms of programmed cell death are well defined, the subsequent immune responses to dying cells in the context of cancer remain relatively unexplored. Necroptosis occurs downstream of the receptor-interacting protein kinases RIPK1 and RIPK3, whose activation leads to lytic cell death accompanied by de novo production of proinflammatory mediators. Here, we show that ectopic introduction of necroptotic cells to the tumor microenvironment promotes BATF3+ cDC1- and CD8+ leukocyte-dependent antitumor immunity accompanied by increased tumor antigen loading by tumor-associated antigen-presenting cells. Furthermore, we report the development of constitutively active forms of the necroptosis-inducing enzyme RIPK3 and show that delivery of a gene encoding this enzyme to tumor cells using adeno-associated viruses induces tumor cell necroptosis, which synergizes with immune checkpoint blockade to promote durable tumor clearance. These findings support a role for RIPK1/RIPK3 activation as a beneficial proximal target in the initiation of tumor immunity. Considering that successful tumor immunotherapy regimens will require the rational application of multiple treatment modalities, we propose that maximizing the immunogenicity of dying cells within the tumor microenvironment through specific activation of the necroptotic pathway represents a beneficial treatment approach that may warrant further clinical development.