CD4+ T Cells Are Required for Production of Disease-Causing Antibodies in an Alloantigen-Specific Murine Model of Fetal/Neonatal Alloimmune Thrombocytopenia

Fetal/Neonatal Alloimmune Thrombocytopenia (FNAIT) occurs when maternal antibodies specific for fetal platelet alloantigens cross the placenta and clear a baby's platelets, causing thrombocytopenia that is unpredictably severe and associated with irreversible brain damage and death. Of the 37 human platelet alloantigens (HPAs) implicated in FNAIT, HPA-1a, which resides on the integrin β3 subunit, is among the most clinically significant. Indirect evidence in humans implicates CD4+ T cells in production of FNAIT-causing HPA-1a-specific antibodies; however, whether CD4+ T cells are required for disease induction, and might therefore be manipulated to prevent disease development, is not known. We employed a novel CRISPR-Cas9-generated humanized transgenic mouse expressing the HPA-1a alloantigen to examine the requirement for CD4+ T cells in development of FNAIT. CD4+ T cells and B cells were purified separately from wild-type BALB/c donor mice that had been immunized, intraperitoneally with adjuvant, with platelets from HPA-1a-transgenic mice, in which HPA-1a-specific B and CD4+ T cells are generated. Donor B cells, either alone or with CD4+ T cells, were adoptively transferred into sub-lethally irradiated Rag1-deficient BALB/c recipient mice. Recipients were immunized with HPA-1a-transgenic platelets, and HPA-1a-specific antibodies were quantified in sequential blood samples collected from recipients. Adoptive transfer of B cells alone from HPA-1a-immunized mice resulted in transient production by Rag1-deficient recipients of only very low levels of HPA-1a-specific antibodies, whereas adoptive transfer of both B and CD4+ T cells resulted in sustained production of high levels of HPA-1a-specific antibodies. These results convincingly demonstrate that, in our mouse model, CD4+ T cells are required for B cell production of the HPA-1a-specific antibodies that cause FNAIT. The phenotypes and functions of CD4+ T cells that are generated in HPA-1a-incompatible pregnancies, and that initiate and amplify production of FNAIT-causing antibodies, are under investigation.

Newman:Rallybio Inc: Consultancy, Research Funding.