Unveiling the truth: different names, same antigen Free

In this issue of Blood, Duval et al¹ are the first to characterize the genetic basis of the Cs^a and Cs^b red blood cell (RBC) antigens, confirming that they correspond to human neutrophil antigen (HNA)-3a and HNA-3b, respectively, both of which are located on the choline transporter-like protein-2 (CTL-2). For many years, the genetic basis of Cs^a and Cs^b was not determined. The current study has removed them from the COST blood group collection of undefined antigens, and now they are included in the International Society of Blood Transfusion (ISBT) nomenclature as ISBT-039.² In addition to RBCs and neutrophils, CTL-2 is expressed on various cell types, including mononuclear blood cells³ and endothelial cells.⁴ 

Duval et al confirmed that both anti-HNA-3 and anti-Cs antibodies specifically reacted with Cs-typed neutrophils, indicating that they target the same antigen. However, anti-HNA-3 antibodies from patients with TRALI (transfusion-related acute lung injury) did not recognize the Cs antigen on RBCs. In contrast, anti-Cs antibodies were able to agglutinate neutrophils in the granulocyte aggregation assay. These findings suggest that the expression of CTL-2 on RBCs and neutrophils may differ structurally. Supporting this, mass spectrometry analysis identified distinct SLC44A2-derived peptides from neutrophil and RBC membranes. Additionally, treatment of neutrophils with dithiothreitol resulted in a significant reduction in anti-HNA-3a antibody binding but did not affect the binding of either anti-Cs^a or anti-HNA-3a antibodies to RBCs. Interestingly, anti-Cs^a antibodies also specifically bound to endothelial cells.

It is likely that anti-Cs^a and anti-HNA-3 antibodies are generated in response to distinct antigenic stimuli: anti-Cs^a by RBCs and anti-HNA-3 by neutrophils. However, differentiating between them remains a challenge. Given that these antibodies have distinct behaviors in RBC recognition, this difference may serve as a potential feature for distinguishing them. Alternatively, unifying the nomenclature under a single term, such as anti-CTL-2, may be more practical than maintaining separate designations for anti-Cs and anti-HNA-3 antibodies; however, this remains a topic of debate. Both antibodies recognize a shared antigen expressed on different cell types, but exhibit varying reactivities due to structural differences in the antigenic molecule across these cell types.

Shared antigens, defined as antigenic groups common to multiple cell types, have garnered increasing attention in recent years. This interest was driven by the identification of CD36 (also known as glycoprotein-IV or Nak-a),⁵ considered an important platelet antigen, as a novel red blood cell antigen, which was officially registered as ISBT-44 in the ISBT Red Cell Immunogenetics and Blood Group Terminology.² The importance of shared antigens lies in the fact that antibodies generated in response to one cell type can provoke clinical conditions by targeting the same antigenic determinant expressed on different cell types. One such clinical manifestation is TRALI, a very severe complication of blood transfusion. TRALI is primarily triggered by antibodies against HLAs, either class I or class II.⁶ These antibodies typically result from prior exposure to incompatible HLA antigens during pregnancy, blood transfusion, or organ or hematopoietic transplantation. Similar to ABO blood group antigens, HLA class I antigens are broadly expressed on various human cell types. When HLA antibodies are produced, they can target and destroy transfused platelets that are incompatible with the recipient, leading to platelet transfusion refractoriness. Additionally, these antibodies bind to the recipient’s white blood cells, triggering neutrophil activation, sequestration, and endothelial barrier damage, which collectively contribute to the development of TRALI.

CTL-2 (ISBT-039)² and CD36 (ISBT-044)² are of particular interest because they are expressed not only on neutrophils, platelets, and red blood cells, but also on monocytes, endothelial cells, and other cell types. Both anti-HNA-3 and anti-CD36 antibodies have been implicated in TRALI. Recent studies have shown that the pathophysiology of anti-CD36 antibody-mediated TRALI is primarily dependent on monocytes and complement activation, rather than neutrophils or platelets.⁷ The interaction between monocytes and endothelial cells in the lungs, activated by tumor necrosis factor-α, enhances CD36 expression on the surface of monocytes, facilitating the binding of anti-CD36 antibodies. This binding triggers complement activation, production of reactive oxygen species (ROS), and cytokine generation, all of which contribute to the development of TRALI.

Two distinct mechanisms of TRALI have been described: a 2-event pathway⁸ and a 1-event pathway.⁹ In the 1-event pathway, anti-HNA-3a antibodies specifically bind to HNA-3a⁺ endothelial cells, leading to ROS production, increased albumin influx, and decreased endothelial resistance in vitro. This pathway is supported by clinical and laboratory evidence of TRALI in mice. Interestingly, this response could be alleviated but not prevented by neutrophil depletion.⁹ In contrast, in the 2-event neutrophil-dependent pathway, HNA-3a antibodies prime neutrophils, leading to neutrophil-mediated endothelial damage. Since neutrophil depletion did not prevent TRALI in their model, it suggests that both pathways are likely involved.

Although anti-Cs antibodies are reported to have limited clinical significance in RBC-incompatible transfusions,¹⁰ their role in TRALI pathophysiology remains unclear. If anti-Cs antibodies do contribute to TRALI, it is uncertain whether they operate mainly through a 2-event pathway involving neutrophils or a direct 1-event pathway targeting endothelial cells. Of note, the current study exclusively used anti-HNA-3 antibodies from patients with TRALI, not from individuals with alloimmune or autoimmune thrombocytopenia, suggesting the need for further exploration. At this juncture, it is important for clinicians to know that although anti-Cs antibodies may not be clinically relevant in the context of red blood cell transfusions, they could still be associated with an increased risk of TRALI.

Conflict-of-interest disclosure: The authors declare no competing financial interests.