GPRC5D is a target for the immunotherapy of multiple myeloma with rationally designed CAR T cells.

Early clinical results of chimeric antigen receptor (CAR) T cell therapy targeting B cell maturation antigen (BCMA) for multiple myeloma (MM) appear promising, but relapses associated with residual low-to-negative BCMA-expressing MM cells have been reported, necessitating identification of additional targets. The orphan G protein-coupled receptor, class C group 5 member D (GPRC5D), normally expressed only in the hair follicle, was previously identified as expressed by mRNA in marrow aspirates from patients with MM, but confirmation of protein expression remained elusive. Using quantitative immunofluorescence, we determined that GPRC5D protein is expressed on CD138+ MM cells from primary marrow samples with a distribution that was similar to, but independent of, BCMA. Panning a human B cell-derived phage display library identified seven GPRC5D-specific single-chain variable fragments (scFvs). Incorporation of these into multiple CAR formats yielded 42 different constructs, which were screened for antigen-specific and antigen-independent (tonic) signaling using a Nur77-based reporter system. Nur77 reporter screen results were confirmed in vivo using a marrow-tropic MM xenograft in mice. CAR T cells incorporating GPRC5D-targeted scFv clone 109 eradicated MM and enabled long-term survival, including in a BCMA antigen escape model. GPRC5D(109) is specific for GPRC5D and resulted in MM cell line and primary MM cytotoxicity, cytokine release, and in vivo activity comparable to anti-BCMA CAR T cells. Murine and cynomolgus cross-reactive CAR T cells did not cause alopecia or other signs of GPRC5D-mediated toxicity in these species. Thus, GPRC5D(109) CAR T cell therapy shows potential for the treatment of advanced MM irrespective of previous BCMA-targeted therapy.

Periosteal hyperostosis (exostosis) in DBA/1 male mice.

Periosteal hyperostosis (exostosis) was identified in 5.9% (11/188) of DBA/1 male mice 10-14 weeks old used for collagen-induced arthritis (CIA) efficacy testing of immunomodulatory biologics. Mice with and without CIA in the affected limb, and also control and treated groups, were involved, with bilateral lesions in one mouse. Hyperostosis was characterized by circumferential and raised masses of variable location, length, and laterality, generally external to but occasionally breaching the periosteum of the metatarsals, metacarpals, tibia, femur, and humerus. Proportionally, the hyperostotic foci consisted of cancellous and woven bone, followed by osteoid, cartilage, and fibrous connective tissue and rarely inflammatory cells. A displaced, presumably pathological fracture with callus formation was a concurrent lesion in only one case. Tartrate-resistant acid phosphatase-positive cells were frequent at bony interfaces, indicating an active resorptive process. Periosteal hyperostosis is an incidental and potentially common finding in DBA/1 mice. Underreporting may occur due to the male bias in disease expression of this CIA model, sampling bias (generally paws only), tissue obliteration in the presence of CIA, and lack of comprehensive historical data on the background and aging lesions in this strain of mouse. Identification of such confounding bony lesions is important to the interpretation of efficacy studies, and suggests the need to further examine the biology of bone development in this strain of mouse.