Alloimmunization against red blood cells (RBCs) is the main immunological risk associated with transfusion in patients with sickle cell disease (SCD). However, about 50-70% of SCD patients never get immunized despite frequent transfusion. In murine models, CD4+ T cells play a key role in RBC alloimmunization. We therefore explored and compared the CD4 + T-cell phenotypes and functions between a group of SCD patients (n = 11) who never became immunized despite a high transfusion regimen and a group of SCD patients (n = 10) who had become immunized (at least against Kidd antigen b) after a low transfusion regimen. We studied markers of CD4 + T-cell function, including TLR, that directly control lymphocyte function, and their spontaneous cytokine production. We also tested responders for the cytokine profile in response to Kidd antigen b peptides. Low TLR2/TLR3 expression and, unexpectedly, strong expression of CD40 on CD4 + T cells were associated with the nonresponder status, whereas spontaneous expression of IL-10 by CD4 + T cells and weak Tbet expression were associated with the responder status.A Th17 profile was predominant in responders when stimulated by Jb k . These findings implicate CD4 + T cells in alloimmunization in humans and suggest that they may be exploited to differentiate responders from nonresponders.Additional supporting information may be found in the online version of this article at the publisher's web-site Previously, we studied regulatory T (Treg) cells, as they are required for the maintenance of self-tolerance and immune homeostasis; despite an altered phenotype in SCD patients, we did not find any difference between Treg-cell phenotypes or functions in alloimmunized and nonalloimmunized patients [11].Different surface markers that could be linked to alloimmunization against RBCs can be used to study the activation state of CD4 + T cells as HLA DR or CD154. Tbet, OX40, and CD40 expressed on CD4 + T cells are also of interest, as these markers have been implicated in the development of autoimmune disease, a clinical phenomenon frequent in SCD patients [12][13][14]. The promotion of Th cells is also dependent on inflammation through direct TLR signaling [15]. Activation of TLR3 or TLR9 can enhance the allogeneic immune responses against RBC antigens in mouse [6,16]. Because of the underlying inflammatory state in SCD, TLR signaling may be central to alloimmunization, but it is not known whether TLR ligands can directly promote T-cell function in patients [17][18][19][20] Understanding the pathophysiology of posttransfusion alloimmunization may allow the identification of early markers of this complication. This may help prevent antibody production by facilitating transfusion of matched RBCs in patients who have a higher risk of immunization and the development of new therapeutic strategies.Given the importance of T cells and T-cell subsets in immune modulation, especially in diseases with antibody production, it is possible that CD4 + T cells in SCD patients may exhibit different phenoty...

Sickle cell disease (SCD) is a chronic inflammatory disease associated with multiple organ damage, chronic anemia, and infections. SCD patients have a high rate of alloimmunization against red blood cells (RBCs) following transfusion and may develop autoimmune diseases. Studies in mouse models have suggested that regulatory T cells (Treg) play a role in alloimmunization against RBC antigens. We characterized the phenotype and function of the Treg cell population in a homogeneous cohort of transfused SCD patients. We found that the distribution of Treg subpopulations differed significantly between SCD patients and healthy blood donors. SCD patients have a particular Treg phenotype, with strong CTLA-4 and CD39 expression and weak HLA-DR and CCR7 expression. Finally, we show that this particular phenotype is related to SCD rather than alloimmunization status. Indeed, we observed no difference in Treg phenotype or function in vitro using autologous feeder cells between strong and weak responders to alloimmunization.

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