Human C-reactive protein does not bind to fcγRIIa on phagocyti cells.
Authors: Terry W. DuClos, Carolyn Mold, Jeffrey C. Edberg, Robert P. Kimberly
Journal: Journal of Clinical Investigation (2001)

We welcome the opportunity to comment on the concerns of Dr. van de Winkel and colleagues about our observations that human FcgRIIa is a receptor for C-reactive protein (CRP).

The original observation that human FcgRIIa is the major receptor for human CRP, reported in the Journal of Experimental Medicine (1), was based on the specific and saturable binding of CRP to COS cells transfected with human FcgRIIa. That observation was supported by the direct immunoprecipitation by immobilized CRP of FcgRIIa from THP-1 cells (see Figure 5 in ref. 1), independent of any antibody-mediated interactions. Further evidence for the role of FcgRIIa as the major receptor for CRP was provided in our paper in the JCI (2). In this work, we demonstrated that, independent of antibody, CRP directly elicits a rise in intracellular Ca2+ levels in polymorphonuclear leukocytes (PMNs) from donors homozygous for the R131 allele of FcgRIIa but not in donors homozygous for the H131 allele (see Figure 4 in ref. 2). Additional evidence that CRP does indeed bind to human FcgRIIa is shown by our recent studies documenting rosetting of CRP-opsonized zymosan by FcgRIIa-transfected COS cells (Figure ​(Figure11 in this letter). In these studies, we have focused on transfection of the R131 allele of FcgRIIa because of the preferential binding of CRP to that allele compared with the H131 allele (2).



Figure 1

COS cells were transfected with FcγRIIa. After 48 hours, cells were washed and incubated with zymosan or CRP-zymosan. After washing, the cells were examined for attached FITC-zymosan. Twenty low-power fields were counted. Results are expressed ...



The concern, expressed by van de Winkel and colleagues, about the use of mAb 2C10 in the detection of CRP binding does raise an appropriate technical caution about murine IgG subclasses and their interaction with the human FcgRIIa H131/R131 polymorphism. We would also raise a second technical caution. In the data provided by van de Winkel and colleagues, there is a striking lack of binding of CRP to human PMNs. Numerous groups over the past 15 years have shown specific, saturable, and antibody-independent binding of CRP to neutrophils (3–8). These binding studies have been supported by functional studies including the demonstration of phagocytosis of CRP-opsonized erythrocytes by neutrophils and inhibition of this activity by modulation of FcgR (9). The inability to detect CRP binding to human PMN with the detection strategy employed by van de Winkel makes interpretation of their data regarding FcgRIIa-transfected COS cells ambiguous.

Multiple lines of evidence indicate that CRP does bind to human FcgRIIa independently of the anti-CRP mAb 2C10. The data provided by van de Winkel do not refute our observations, but they do underscore the need for careful attention to technical details in working with CRP. We would also emphasize that CRP binding is not limited to human FcgRs. Murine FcgR have the same capacity (10–12). Binding studies using various FcgR-deficient mouse strains identified FcgRI and FcgRII as CRP receptors in the mouse and showed that cells from mice lacking all FcgR do not bind CRP (12). These results were confirmed by phagocytosis assays using CRP-opsonized zymosan (13).