Identification of Immunodominant Alloreactive T-Cell Epitopes on the Jk^a Red Blood Cell Protein Inducing Either Th1 or Th2 Cytokine Expression: Role of DRB1*01 Molecules in Jk^a Peptide Presentation.

Background: although antibodies implicated in red blood cell (RBC) blood group antigen alloimmunization are studied for many years, little is known about helper T cell responses that drive their production. The aim of this work was to determine T cell antigenic determinants involved in T cell responses against RBC antigens, and the subsequent patterns of stimulatory cytokine production.

Study Design: the Kidd blood group antigen (Jk^a), was selected as a model. We investigated cytokine expression by helper T cells after stimulation by Jk^a peptides. Peripheral mononuclear cells were isolated from Jk^a primed donors producing anti-Jk^a antibodies and from non anti-Jk^a alloimmunized donors as negative controls. Four 16-mer peptides mimicking the Jk^a sequence, 266–293, including the Asp280 polymorphism, and overlapping each over by 12 amino acids were tested to map alloreactive T cell epitopes. A sensitive method, the real time RT-PCR, was chosen to quantify Th1- and Th2-type cytokines (respectively IL-2 and IL-4) after Jk^a peptide stimulations. DRB1* and DQB1* low resolution typing, and DRB1* DNA subtyping were performed.

Results: lymphocytes from non anti-Jk^a alloimmunized donors produced low level of IL-2 or IL-4 (<10copies), while lymphocytes from all anti-Jk^a alloimmunized donors responded in a much higher level to at least one of the four Jk^a peptides. A clear Th1/Th2 dichotomy in the cytokine response was observed in this population. Indeed, among the 10 anti-Jk^a alloimmunized donors tested, 4 produced IL-2 alone and 6 produced IL-4 alone. Sequences among two peptides (Jk^a1and Jk^a2) were together able to elicit a response (IL-2 or IL-4) in 8 among 10 of these anti-Jk^a alloimmunized donors and represent a pool of immunodominant peptides. This Th1/Th2 dichotomy was not due to delays in kinetics of IL-2 and IL-4 productions. It was neither related to particular donor DRB1* or DQB1* molecules. The DRB1*01 phenotype frequency was increased (82%) in the anti-Jk^a alloimmunized donors as compared to the expected frequency (18%) in the caucasian population. This observation raised the question as to whether these molecules are associated to genetic susceptibility to anti-Jk^a alloimmunization.

Conclusion: These data indicate that Th1 and Th2 subsets are associated to the specific memory humoral immune response against Jk^a RBCs. The cytokine pattern (IL-2 or IL-4) is characteristic of the donor, whatever the peptide tested. DRB1*01 seems to be implicated in the peptide presentation to the T lymphocyte.

A detailed understanding of peptides and cytokines involved in T cell responses to Jk^a protein and the HLA class II molecules implicated in the peptide presentation constitutes a first step toward evaluation of peptide immunotherapy to prevent or treat anti-RBC alloimmunization.