Deciphering of CD4⁺ T Cell Subsets Involved in Maintenance and Break of Immune Tolerance in a Humanized Mouse Model That Is Immune Tolerant to Human FVII

Abstract 2188

25%-30% of patients with hemophilia A develop neutralizing antibodies following replacement therapy with factor VIII (FVIII). These patients can be treated with factor VIIa (FVIIa) which triggers the extrinsic pathway of coagulation and thereby bypasses the requirement for FVIII. We developed a new mouse model that is transgenic for human FVII and expresses specific immune tolerance to native human FVIIa. We aim to investigate the immunological impact of modified FVIIa product candidates and to characterize their immunogenicity by analyzing emerging FVIIa-specific T cell responses.

The new mouse model offers a unique opportunity to study central and peripheral immune regulatory mechanisms and the generation of immune responses by pro-inflammatory antigen-specific effector T cells (Teff). We hypothesized that FVIIa-specific Teff having escaped clonal deletion are present in the periphery and may be actively suppressed by FVIIa-specific regulatory T cells (Tregs). To study this hypothesis, we immunized mice with recombinant FVIIa (rFVIIa) with or without LPS, a well-described “danger signal” being able to break immune tolerance by stimulating the innate immune system. Intravenous or subcutaneous administration of rFVIIa alone did not elicit antibody responses and thus immune tolerance to rFVIIa was not broken. However, co-administration of rFVIIa and LPS resulted in a specific antibody response that was not isotypically restricted. To further analyze the mechanisms behind this break of specific immune tolerance, we characterized rFVIIa-specific T cells by the expression of CD154, a marker of antigen-specific T cells. Cytokine production and CD154 expression were assessed upon re-stimulation with rFVIIa. In contrast to mice that were immunized with rFVIIa only, we found increased numbers of rFVIIa-specific T cells in rFVIIa-LPS-treated mice displaying a stable, highly pro-inflammatory (IL-2⁺/IFN-g⁺) memory phenotype.

These data could suggest that rFVIIa-specific Teff that escaped clonal deletion during induction of central immune tolerance, are present in the periphery of human FVII-transgenic mice. This would imply that rFVIIa-specific Teff could be actively suppressed by Tregs. This suppression could be overcome by danger signals like LPS. We currently study the regulatory mechanisms that maintain tolerance upon administration of FVIIa without LPS. We are approaching this question by correlating the characteristics of FVIIa-specific Teff and Treg responses under both tolerant and non-tolerant conditions.

Ultimately, we aim to understand which danger signals have to be provided to break immune tolerance and how tolerance is regulated. Understanding these regulatory mechanisms will enable us to develop new therapeutic strategies and prevent conditions that lead to the induction of antibodies against drug products in patients.