Subcutaneous Allergen Immunotherapy With Hypoallergenic Bet v 1 Compared to Conventional Extract: Poorer Blocking Antibody Capacity Dominated by IgG1 Instead of IgG4

Background: Hypoallergenic recombinant fold-variants of major allergens have been suggested as safer and more effective AIT candidates. The Bet v 1-fold variant BM41, with confirmed preclinical hypoallergenicity and increased immunogenicity, was proposed for the treatment of birch pollen allergy. Methods: We performed a 6-month randomized, double-blind, placebo-controlled first-in-human clinical trial with BM41, a licensed birch pollen extract-based treatment, as the active comparator (AC), and placebo (n = 16, n = 16, and n = 15, respectively). The primary endpoint was safety. Secondary outcomes were Bet v 1-specific (s)IgE, IgG, IgG1, and IgG4 responses measured by ImmunoCAP, and sIgE-blocking activity using mediator release and facilitated antigen binding assays. Results: Despite SPT-confirmed hypoallergenicity (~50% compared to natural Bet v 1), more adverse events occurred in response to BM41. Although similar sIgG and sIgG1 levels were induced, sIgG4 levels increased 3-fold more in AC compared to the BM41 group. In AC, the sIgG4/sIgG1 ratio tripled over time, whereas for BM41 it stagnated. BM41 induced efficient serum inhibitory activity for sIgE compared to placebo but was 12%–32% less efficient than AC. Both sIgG4 and sIgG1 contributed to the blocking effect in AC, while in BM41 both sIgG subclasses showed a lowered functional capacity. Conclusion: Preclinically established hypoallergenicity of BM41 did not result in a lower number of adverse events. The reduced induction of sIgG4 by the fold variant in the course of the treatment was less efficient in blocking sIgE-mediated responses. This is the first study providing evidence that, instead of a Th1-favored IgG1-dominated response, “modified Th2”-skewed IgG4-dominated humoral responses are beneficial in AIT vaccine design.