FcγRIIIa role in rituximab efficacy

The gene encoding FcγRIIIa (CD16), an activating Fc receptor expressed primarily on natural killer (NK) cells, has 2 allelic single-nucleotide polymorphic (SNP) variants, valine (V) or phenylalanine (F), at position 158. The 158V SNP exhibits a higher binding affinity for human immunoglobulin G₁ (IgG₁) than the 158F SNP, and higher levels of antibody-dependent cellular cytotoxicity (ADCC) have been observed in NK cells from homozygous V/V individuals. In 2000, Clynes et al¹  reported that mice genetically deficient in FcγR had greatly diminished response to therapy with rituximab, an anti-CD20 monoclonal antibody used in the treatment of B-cell malignancies. Other xenograft NK-cell depletion studies² ,³  have confirmed these findings.

As a consequence of this ongoing research, it has gradually become accepted that Fcγ- receptor–mediated immune mechanisms contribute substantially to the mechanisms of action of antitumor antibodies. Consequently, investigators began to examine the relationship between the specific FcγRIIIa genotype and clinical outcome in patients receiving antitumor antibodies. Cartron et al⁴  demonstrated that non-Hodgkin lymphoma (NHL) patients carrying the 158V/V genotype had significantly improved objective response rates as compared with patients carrying at least one 158F allele, further supporting the role of FcR-mediated effector functions in general, and NK cells in particular, in the antitumor effects of rituximab in NHL.

The association between FcγRIIIa genotype and clinical outcome generally has been attributed to differences in IgG binding affinity. However, in the current report, Hatjiharissi and colleagues quantitatively measured the number of FcγRIIIa molecules expressed on the NK-cell surface in donors of different FcγRIIIa genotypes. These investigators found that NK cells from individuals carrying at least one 158V allele had significantly higher cell-surface expression of FcγRIIIa than individuals possessing two 158F alleles. NK-cell antibody–dependent cellular cytotoxicity (ADCC) against rituximab-coated target cells also correlated with FcγRIIIa expression levels, with the NK-cell donors expressing the highest number of CD16 molecules per NK cell having the highest ADCC activity. In confirmation of previous reports, the authors use a competition assay to demonstrate that NK cells from 158V/V donors bind rituximab more strongly than 158V/F or 158F/F NK cells, an effect which is independent of the number of receptors expressed on the cell surface. Although the study is somewhat limited by the low number of donors examined, it is likely that both mechanisms contribute to the higher clinical benefit rates observed in 158V/V patients receiving rituximab. Further mechanistic studies of context-identical genotyped cells will be required to determine “where the beef” is, relative to rituximab efficacy and the importance of FcγRIIIa affinity versus FcγRIIIa-receptor expression on NK cells.

The importance of the findings put forth by Hatjiharissi and colleagues are substantial, and relate to both alternative effector-cell contribution to antibody therapy and to novel ways NK-cell ADCC might be augmented. Clynes et al¹  have shown that genetic deletion of FcγRIIb, an inhibitory Fc receptor expressed on monocytes, significantly improves the antitumor response to rituximab. Other reports have also demonstrated that the effects of anti-CD20 mAbs are substantially reduced in mice depleted of monocytes.⁵  These reports suggest that monocytes/macrophages might also contribute to the clearance of B cells following rituximab administration. Consistent with this hypothesis, a genetic polymorphism in FcγRIIa (histidine/arginine at position 131), an activating receptor expressed on monocytes, has been correlated with clinical response to rituximab. However, unlike the case for the FcγRIIIa 158 dimorphism, the FcγRIIa allotypes exhibit similar binding activity to rituximab, and therefore it is currently unclear how the dimorphism influences clinical response to rituximab. It will therefore be important to conduct similar studies investigating the relationship between the FcγRIIa genotype and receptor-expression levels. Such a correlation, if found, may explain the association of the 131H/H polymorphism with greater response to rituximab.

With respect to targeting NK cells, these findings suggest that the coadministration of immune modulatory adjuvants known to up-regulate FcγRIIIa expression may enhance rituximab-specific NK-cell activity in NHL patients, thereby improving clinical outcome. These studies thereby strongly support the use of NK-cell–activating adjuvants in combination with rituximab in NHL patients.