Imprime PGG (Imprime), a soluble yeast 1,3/1,6 β-glucan, is being developed as a novel cancer immunotherapy in conjunction with anti-tumor antibodies in several cancers. Randomized Phase 2 clinical trials of Imprime in the 1st-line treatment of stage IV non-small cell lung cancer have shown promising efficacy in terms of both objective tumor response and survival. Mechanistic studies have demonstrated that Imprime forms an immune complex with endogenous IgG and IgM anti-β-glucan antibodies (ABA). The immune complex then binds to and primes innate immune cells, including macrophages, monocytes and neutrophils, to kill antibody-targeted cancer cells via a complement receptor 3-dependent mechanism.
In this study, we sought to investigate how the formation of this Imprime-ABA immune complex might influence patient selection strategies. We have shown that there is a threshold level of ABA required for binding of Imprime-ABA immune complex to the innate immune effectors. In human healthy volunteers (N=143) IgG ABA concentrations vary from 8 to 1448 μg/ml. Imprime treatment of whole blood from individuals with ABA concentrations above a threshold yields immune complex formation that allows both classical and alternative complement pathways leading to complement opsonization of the complex. This complex then binds innate immune cells, triggering the modulation of cell surface receptors and production of selective chemokines such as IL-8.
We now show that the predominant subclass of endogenous IgG ABA specific to Imprime is IgG2. Additionally, we show that binding of Imprime to immune cells critically involves FcgRIIA, the only Fc receptor capable of binding IgG2 immune complexes. Blocking FcgRIIA significantly inhibits binding to both neutrophils and monocytes in whole blood. This observation was further confirmed by binding of Imprime to HEK cells overexpressing FcgRIIA. In order to evaluate the effect that ABA IgG subclass has on Imprime, chimeric ABAs with identical affinity but differing in the human IgG subclass (IgG1 vs. IgG2) were generated using the heavy and light chain variable domains from a mouse ABA hybridoma. IgG1 or IgG2 ABA added to whole blood of healthy donors with low ABA levels was able to increase Imprime binding, consistent with previous data showing that Imprime binding is dependent on ABA levels. However, the relative increase in binding due to the addition of IgG1 or IgG2 ABA varied among donors. While addition of either IgG1 or IgG2 ABA was equivalent in increasing binding in some donors, IgG1 was more efficacious than IgG2 in other donors. These results led us to look at allelic variants of the FCGR2A gene.
In humans, there is a SNP in FCGR2A, R131H, that defines two alleles whose protein products differ in affinity for IgG2, with H variant designating the higher affinity allele and the R variant indicating lower affinity. The variable binding of Imprime in donors with addition of chimeric ABA was found to be related to the FCGR2A genotype. Individuals homozygous for the R genotype showed lower binding with IgG2 chimeric ABA than with IgG1 chimeric ABA. The findings were similar with exogenously added naturally occurring ABA purified from serum. The effect of the R131H SNP on Imprime binding in relation to endogenous ABA levels was further confirmed in a survey of 140 healthy volunteers. The prevalence and functional relevance of this SNP on Imprime binding to immune cells in cancer patients is being evaluated. Together, these data implicate both the concentration of ABA in serum and FCGR2A genotype as critical determinants of the ability of an individual to bind to Imprime and suggest that either or both may enable the selection of patients most likely to derive therapeutic benefit from Imprime treatment.
Citation Format: Nandita Bose, Anissa Chan, Adria Jonas, Nadine Ottoson, Xiaohong Qiu, Lindsay Wurst, Steven Leonardo, Peter Maimonis, Katie Ertelt, Ben Harrison, Keith Gorden. Endogenous anti-β-glucan antibodies and FcgRIIA (CD32a) single nucleotide polymorphisms (SNP) as potential predictive biomarkers for the efficacy of Imprime PGG immunotherapy in cancer patients. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr A014.
- ©2016 American Association for Cancer Research.