Engineered Flt3L Drives Tolerogenic State to Attenuate Anti-drug Antibody Responses

Immune reactions to protein drugs present substantial challenges to protein replacement for treating congenital diseases and metabolic deficiencies, due to the lack of endogenous tolerance or the protein drug's partial or total non-human origin. We sought to transiently modify the immune environment when the adaptive response to the drug antigen is mounted to lessen future reactions upon continued therapeutic treatment, without modifying the drug itself. Herein, we characterize a recombinant fusion of the cytokine Flt3L to serum albumin and describe a novel pathway of Flt3L-mediated immune regulation. We highlight reduced activation of dendritic cells (DC) as well as an increased frequency of DCs expressing LAP, a TGF-β precursor. These effects in combination with low doses of the exogenous antigen led to less TH2 differentiation. This enabled a tolerance-biasing induction regimen to significantly decrease anti-drug antibodies upon repeated exposure to a clinically used, immunogenic fungal enzyme, rasburicase. This induction regimen reduced the Tfh compartment and increased Tfh cells expressing Foxp3 and PD-L1, suggesting a regulatory response. Overall, we introduce the use of a Flt3L variant as an induction therapeutic to modulate the innate immune response, thereby attenuating the adaptive reaction to antigenic protein drugs and addressing an unmet clinical need. ### Competing Interest Statement A.T.A., R.P.W., K.C.R., A.L.L., and J.A.H. are all listed as inventors on patents related to Flt3L-SA use for therapeutic purposes filed by the University of Chicago. Other authors declare no competing interests.