Background/Purpose:   Our proprietary variant Ig domain (vIgD) platform creates novel, therapeutically-applicable protein domains with tailored specificity and affinity. These vIgDs are created through directed evolution of immunoglobulin superfamily (IgSF) proteins, which are key components of the immune system that include well-known family members such as PD-1, PD-L1, and CTLA-4. CD28 and Inducible T-cell Costimulator (ICOS) are two related costimulatory molecules within the IgSF which are expressed on T cells and interact with CD80/CD86 and ICOS ligand (ICOSL), respectively. Both play critical roles in T cell activation and adaptive immunity. We have used our vIgD platform to generate human ICOSL-Fc vIgDs capable of binding both ICOS and CD28, blocking the interaction of these costimulatory molecules with their respective receptors.  

Methods:   ICOSL-Fc molecules were evaluated in vitro in mixed lymphocyte reactions (MLR) generated using negatively-selected human pan T cells mixed with activated human monocyte-derived dendritic cells, and in vivo in standard mouse models of delayed type hypersensitivity (DTH), collagen-induced arthritis (CIA), and human-mouse xenograft PBMC-NSG^TM graft versus host disease (GvHD).

Results:   ICOSL-Fc fusion proteins containing variant ICOSL domains significantly attenuate T cell activation in vitro as assessed by suppressed proliferation and cytokine production in MLR. They also reduce mouse DTH reactions in vivo. ICOSL-Fc molecules mediate significant disease reduction, matching or exceeding CTLA-4-Ig comparators targeting only the CD28 pathway in mouse CIA, and in the human PBMC-NSG^TM GvHD model. ICOSL-Fc suppresses the production of anti-collagen antibodies in CIA, likely reflecting the key roles CD28 and ICOS play in follicular helper T cell differentiation and T-dependent antibody responses. 

Conclusion:   Efficacy in vitro and in vivo of ICOSL-Fc is superior to wild-type ICOSL domains due to the induced alterations in affinity for cognate ligand (ICOS) and through specifically directed changes in ICOSL-Fc’s ability to bind additional counter-structures (i.e. CD28). Thus, vIgDs like these ICOSL variants can be developed to acquire unique biochemical properties that can potentially significantly enhance their therapeutic utility as immunomodulatory agents. This vIgD therapeutic platform has broad potential to enhance the activity of biologics in treatment of autoimmune and other disorders driven or subject to modulation by IgSF proteins, such as cancer and infectious diseases. Preclinical development of ICOSL-Fc has been initiated to support clinical studies.