Gene Therapy for Hemophagocytic Lymphohistiocytosis (HLH): Fixing a Criticial ‘Circuit Breaker’ in the Immune System.
Authors: Jordan Michael , Carmo Marlene , Tiwari Swati , Arumugam Paritha , Risma Kimberly , Gaspar Bobby , Malik Punam 
Published: October 2019

Abstract
Abstract 3158
In addition to its well-known role in host defense, perforin-dependent cytotoxicity also has a critical immune regulatory function. Genetic defects of perforin underlie many cases of hemophagocytic lymphohistiocytosis (HLH), a fatal disorder of extreme immune activation. While the initial treatment of HLH is largely focused on suppressing dangerous inflammation, hematopoietic cell transplantation (HCT) is required for long-term correction of the underlying immune regulatory defect. Unfortunately, mortality after HCT in patients with HLH is unusually high for a non-malignant disorder, ranging from 20 to 50%.
We developed a model of HLH in which perforin deficient (prf-/-) mice develop all of the features of HLH after challenge with lymphocytic choriomeningitis virus (LCMV). Recent studies from our group have demonstrated that transplantation of wild type (WT) bone marrow into prf-/- mice rescues them from the subsequent development of HLH after challenge with LCMV. In this model, perforin-dependent immune regulation appears to function in trans, with a critical threshold of 10% WT chimerism required to protect animals from HLH. This low threshold is consistent with clinical reports of very limited numbers of patients with long term mixed chimerism after HCT who appear to be protected from HLH recurrence. These data have also provided a clear target for gene therapy efforts.
We have begun to test the feasibility of autologous gene correction using lentiviral vectors in this model system. Hematopoietic stem and progenitor cells (HSC) from prf-/- mice were transduced with lentivirus vectors expressing perforin under the control of ubiquitous or tissue-specific promoters. When transduced HSC were transplanted into lethally irradiated prf-/- mice, we have observed levels of in vivo gene correction ranging from 10–80%, depending on the promoter and transduction protocols utilized. We find that the transduced CD8 T cell and NK cell progeny display significant correction of their cytotoxic defects. Challenge of gene corrected prf-/- mice with LCMV has revealed significant correction of the underlying immune regulatory defect and protection from the development of HLH. However, reestablishing normal perforin-dependent immune regulation with gene corrected prf-/- HSC's appears to be somewhat less efficient than with mixed WT:prf-/- chimerism. Thus, further optimization of perforin gene control by lentiviral vectors appears to be warranted. Results of ongoing studies related to vector optimization and reestablishing physiologic perforin-mediated immune regulation will be presented.
Because normal perforin expression in only a fraction of lymphocytes is sufficient to rescue individuals from HLH, this disorder is an attractive target for gene therapy approaches. Demonstration of long term gene correction and rescue from HLH in our animal model will pave a path for translational efforts testing gene therapy for HLH in clinical trials.

Disclosures:
No relevant conflicts of interest to declare.