Background/Purpose: CD8 Treg, characterized in human peripheral blood mononuclear cells (PBMC) by expression of inhibitory killer immunoglobulin receptors (KIRs), regulate immune balance by eliminating self-reactive and potentially pathogenic CD4 T cells. Control of pathogenic CD4 T cells appears impaired in patients with rheumatoid arthritis and systemic lupus erythematosus (SLE), in part due to reduced CD8 Tregs prevalence and functions. Animal models of rheumatologic diseases suggest that increasing the prevalence of CD8 Treg may correct immune system imbalance and reduce autoimmune disease pathology and progression.

Methods: IL-15 is a pleiotropic cytokine that results in activation and proliferation of various lymphocyte populations. IL-15 expands CD8 Treg effectively in vitro, but also expands conventional CD8 and NK cells. To limit the effects of IL-15 to CD8 Treg, we developed an antibody tethered IL-15 mutein. Point mutations at IL-15 residues that interact with the IL-15 receptor chains can reduce binding affinity between the IL-15 mutein and the IL-15 receptor complex, resulting in a mutein that has greatly reduced potency across all cell subsets. Fusion of the IL-15 mutein to a cell specific targeting antibody restores IL-15 mutein functions only in select immune cell subsets.

Results: In this study, we investigated the selective activation and expansion of CD8 Treg cells using a CD8 Treg-targeted IL-15 mutein in vitro and in vivo. The mutein demonstrated minimal activity in potentially pathogenic cell populations while effectively expanding CD8 Treg from patients with SLE, RA and Sjogren’s in vitro. RNA and TCR sequencing confirmed that IL-15 mutein expanded the same populations of CD8 Treg cells as wild-type IL-15, and maintained CD8 Treg functional capacity of pathogenic CD4 cells elimination in vitro.

In vivo experiments using C57BL/6 mice showed that a targeted IL-15 mutein selectively expanded the CD8 Treg population by thirty-fold within 4 days, with sustained expansion detected up to 14 days after a single dose. Experiments using NSG mice engrafted with human PBMCs to cause a rapid onset of highly inflammatory acute graft versus host disease (GvHD), confirmed a five-fold increase in CD8 Treg eight days after targeted IL-15 mutein administration, compared to equimolar non-targeted IL-15 mutein administration or untreated mice.

Conclusion: Our data show that a targeted low affinity IL-15 mutein can be used to selectively activate and expand a functional CD8 Treg population in vitro and in vivo.  Our data support the therapeutic potential of this approach to selectively increase the prevalence of a functional CD8 Treg population in rheumatologic disease patients that have a reduced prevalence of CD8 Treg and to ameliorate disease.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/targeted-il-15-muteins-provide-selective-expansion-of-kir-cd8-regulatory-t-cells-with-the-potential-to-ameliorate-disease-in-autoimmune-patients-with-deficient-cd8-treg-populations/