Antibodies to Senescent Antigen and C3 Are Not Required for Normal Red Blood Cell Lifespan in a Murine Model

Erythropoietin downregulates red blood cell clearance, increasing transfusion efficacy in severely anemic recipients

Red blood cells (RBC) transfusion is used to alleviate symptoms and prevent complications in anemic patients by restoring oxygen delivery to tissues. RBC transfusion efficacy, that can be measured by a rise in hemoglobin (Hb) concentration, is influenced by donor-, product-, and recipient-related characteristics. In some studies, severe pre-transfusion anemia is associated with a greater than expected Hb increment following transfusion but the biological mechanism underpinning this relationship remains poorly understood. We conducted a prospective study in critically ill patients and quantified Hb increment following one RBC transfusion. In a murine model, we investigated the possibility that, in conjunction with the host erythropoietic response, the persistence of transfused donor RBC is improved to maintain a highest RBC biomass. We confirmed a correlation between a greater Hb increment and a deeper pre-transfusion anemia in a cohort of 17 patients. In the mouse model, Hb increment and post-transfusion recovery were increased in anemic recipients. Post-transfusion RBC recovery was improved in hypoxic mice or those receiving an erythropoiesis-stimulating agent and decreased in those treated with erythropoietin (EPO)-neutralizing antibodies, suggesting that EPO signaling is necessary to observe this effect. Irradiated recipients also showed decreased post-transfusion RBC recovery. The EPO-induced post-transfusion RBC recovery improvement was abrogated in irradiated or in macrophage-depleted recipients, but maintained in splenectomized recipients, suggesting a mechanism requiring erythroid progenitors and macrophages, but which is not spleen-specific. Our study highlights a physiological role of EPO in downregulating post-transfusion RBC clearance, contributing to maintain a vital RBC biomass to rapidly cope with hypoxemia.

Warm autoimmune hemolytic anemia: new insights and hypotheses

PURPOSE OF REVIEW

Warm autoimmune hemolytic anemia (wAIHA) is the most common of the immune hemolytic anemias. Although there are numerous case reports and reviews regarding this condition, some of the unusual and more recent findings have not been fully defined and may be contentious. This review will provide insight into the common specificity of the warm autoantibodies and hypothesize a novel mechanism of wAIHA, that is proposed to be linked to the controversial subject of red blood cell senescence.

RECENT FINDINGS AND HYPOTHESES

It is now well established that band 3 on the red blood cell is the main target of autoantibodies in wAIHA. wAIHA targets the older red blood cells (RBCs) in about 80% of cases and, recently, it has been shown that the RBCs in these patients are aging faster than normal. It has been proposed that in these 80% of patients, that the autoantibody recognizes the senescent red blood cell antigen on band 3. It is further hypothesized that this autoantibody's production and potency has been exacerbated by hypersensitization to the RBC senescent antigen, which is processed through the adaptive immune system to create the pathogenic autoantibody. Recent publications have supported previous data that the senescent RBC antigen is exposed via a dynamic process, wherein oscillation of a band 3 internal loop flipping to the cell surface, creates a conformational neoantigen that is the RBC senescent antigen. It has also recently been shown that the cytokine profile in patients with wAIHA favors production of inflammatory cytokines/chemokines that includes interleukin-8 which can activate neutrophils to increase the oxidative stress on circulating RBCs to induce novel antigens, as has been postulated to favour exposure of the senescent RBC antigen.

SUMMARY

This manuscript reviews new findings and hypotheses regarding wAIHA and proposes a novel mechanism active in most wAIHA patients that is due to an exacerbation of normal RBC senescence.

Rapid clearance of storage-induced microerythrocytes alters transfusion recovery

Permanent availability of red blood cells (RBCs) for transfusion depends on refrigerated storage, during which morphologically altered RBCs accumulate. Among these, a subpopulation of small RBCs, comprising type III echinocytes, spheroechinocytes, and spherocytes and defined as storage-induced microerythrocytes (SMEs), could be rapidly cleared from circulation posttransfusion. We quantified the proportion of SMEs in RBC concentrates from healthy human volunteers and assessed correlation with transfusion recovery, investigated the fate of SMEs upon perfusion through human spleen ex vivo, and explored where and how SMEs are cleared in a mouse model of blood storage and transfusion. In healthy human volunteers, high proportion of SMEs in long-stored RBC concentrates correlated with poor transfusion recovery. When perfused through human spleen, 15% and 61% of long-stored RBCs and SMEs were cleared in 70 minutes, respectively. High initial proportion of SMEs also correlated with high retention of RBCs by perfused human spleen. In the mouse model, SMEs accumulated during storage. Transfusion of long-stored RBCs resulted in reduced posttransfusion recovery, mostly due to SME clearance. After transfusion in mice, long-stored RBCs accumulated predominantly in spleen and were ingested mainly by splenic and hepatic macrophages. In macrophage-depleted mice, splenic accumulation and SME clearance were delayed, and transfusion recovery was improved. In healthy hosts, SMEs were cleared predominantly by macrophages in spleen and liver. When this well-demarcated subpopulation of altered RBCs was abundant in RBC concentrates, transfusion recovery was diminished. SME quantification has the potential to improve blood product quality assessment. This trial was registered at www.clinicaltrials.gov as #NCT02889133.

How Do Red Blood Cells Die?

Normal human red blood cells have an average life span of about 120 days in the circulation after which they are engulfed by macrophages. This is an extremely efficient process as macrophages phagocytose about 5 million erythrocytes every second without any significant release of hemoglobin in the circulation. Despite large number of investigations, the precise molecular mechanism by which macrophages recognize senescent red blood cells for clearance remains elusive. Red cells undergo several physicochemical changes as they age in the circulation. Several of these changes have been proposed as a recognition tag for macrophages. Most prevalent hypotheses for red cell clearance mechanism(s) are expression of neoantigens on red cell surface, exposure phosphatidylserine and decreased deformability. While there is some correlation between these changes with aging their causal role for red cell clearance has not been established. Despite plethora of investigations, we still have incomplete understanding of the molecular details of red cell clearance. In this review, we have reviewed the recent data on clearance of senescent red cells. We anticipate recent progresses in in vivo red cell labeling and the explosion of modern proteomic techniques will, in near future, facilitate our understanding of red cell senescence and their destruction.

Humanized Rodent Models for Cancer Research

As one of the most popular laboratory animal models, rodents have been playing crucial roles in mechanistic investigations of oncogenesis as well as anticancer drug or regimen discoveries. However, rodent tumors show different or no responses to therapies against human cancers, and thus, in recent years, increased attention has been given to mouse models with xenografted or spontaneous human cancer cells. By combining with the human immune system (HIS) mice, these models have become more sophisticated and robust, enabling in vivo exploration of human cancer immunology and immunotherapy. In this review, we summarize the pros and cons of these humanized mouse models, with a focus on their potential as an in vivo platform for human cancer research. We also discuss the strategies for further improving these models.