Renal transplantation represents the optimal treatment for most patients with end-stage renal failure. Although there has been an incremental reduction in the severity and frequency of acute T cell-mediated (cellular) rejection, acute and chronic antibody-mediated rejection (AMR) remain challenging 1,2. The occurrence and severity of antibody-mediated pathology is variable, and it is likely that genetic polymorphisms that affect the magnitude of the B cell response and of the effector functions of antibody in the recipient give rise to such pathological variation. An additional unresolved challenge to long-term allograft survival is that of recipient death with a functioning graft. This occurs most frequently in the context of infection, malignancy or cardiovascular disease, all of which are influenced heavily by immunological factors.
Many effector functions of antibody are mediated by a family of receptors (FcγRs) that are expressed on the majority of immune cells, including neutrophils, natural killer (NK) cells and B cells. The activating effects of immunoglobulin (Ig)G on these myeloid cells are controlled by a single inhibitory receptor, FcγRIIB (CD32B). FcγRIIB is also expressed by B cells and plasma cells, regulating B cell activation following encounter with immune-complexed antigen 3,4. Thus, the inhibitory IgG receptor FcγRIIB plays a critical role in controlling both antibody generation and its immune activating and inflammatory effects. FcγRIIB-deficient mice are prone to inducible and spontaneous antibody-associated autoimmune disease 5,6, but have heightened cytotoxic responses to tumours 7 and are protected from some infections 8,9. In murine cardiac allograft models FcγRIIB had no effect on acute allograft rejection, but chronic arteriopathy and autoantibody production were increased in FcγRIIB-deficient recipients 10.
In humans, a single nucleotide polymorphism (SNP, rs1050501) has been identified in the FCGR2B gene that encodes an amino acid substitution (a threonine for an isoleucine at position 232) within the transmembrane domain of the receptor. FcγRIIB-T232 is associated with receptor dysfunction 11,12 and is found at increased frequency in patients with systemic lupus erythematosus (SLE) 13. The prevalence of this polymorphism shows considerable racial variation (7–13% of Africans are homozygous for FcγRIIB-T232 but only 1–2% of Caucasians 13), which may have arisen due to enhanced protective immune responses to some pathogens in FcγRIIB-T232 homozygotes 9,11,13. We sought to determine the effect of the FCGR2B SNP on outcomes in renal transplantation 14.
The FCGR2B SNP rs1050501 was genotyped in three cohorts of renal transplant recipients enrolled into the Collaborative Transplant Study; cohort A comprised 2851 Caucasian patients; cohort B, 570 African Caribbean patients; and cohort C, 236 patients with a primary diagnosis of SLE to determine whether rs1050501 affected allograft or patient survival.
In cohort A the frequency of FcγRIIB-T232 homozygotes was 2·2% and in cohort B was 6·8% (Table 1), consistent with published data for Caucasian and African control populations 13.
Table 1.
Patient demographics and rates of rejection during first year post-transplant
| Cohort A (Caucasian) | Cohort B (African Caribbean) | Cohort C (SLE) | |
|---|---|---|---|
| Number of patients | 2851 | 570 | 236 |
| Median follow-up (years) | 7 | 3·6 | 7 |
| HLA mismatch | |||
| 0–1 | 419 (14·7%) | 24 (4·2%) | 49 (20·8%) |
| 2–4 | 2031 (71·2%) | 322 (56·5%) | 156 (66·1%) |
| 5–6 | 401 (14·1%) | 224 (39·3%) | 31 (13·1%) |
| FCGR2B genotype | |||
| I/I | 2219 (77·8%) | 319 (56·0%) | 188 (79·7%) |
| I/T | 568 (19·9%) | 212 (37·2%) | 39 (16·5%) |
| T/T | 64 (2·2%) | 39 (6·8%) | 9 (3·8%) |
| Rejection during first year | |||
| I/I | 273 (25·1%) | 43 (29·7%) | 19 (25·3%) |
| I/T | 65 (22·4%) | 33 (29·7%) | 1 (5·9%) |
| T/T | 9 (29·0%) | 3 (15·8%) | 0 (0·0%) |
| P-value | 0·51 | 0·49 | 0·24 |
SLE = systemic lupus erythematosus (reproduced from 14, copyright 2014, with permission from Wolters Kluwer Health). HLA = human leucocyte antigen.
In cohort A, the proportion of patients with a pre-transplant panel-reactive antibody (PRA) of > 10% was highest in the FcγRIIB-T/T232 genotype group (31·1%) versus 24·2 and 23·9% in the subjects with the FcγRIIB-T/I232 and FcγRIIB-I/I232 genotypes, respectively, but this did not reach statistical significance. The frequency of treatment for rejection was also highest in FcγRIIB-T/T232 patients [29% (Table 1)] but, again, this was not statistically significant.
Death-censored allograft survival did not differ significantly between FCGR2B genotypes either at 1 year (93·6, 92·9 and 91·1% in those with FcγRIIB-T/T232, FcγRIIB-T/I232 and FcγRIIB-I/I232 genotypes, respectively), 5 years (79·2, 85·5 and 81·5%, respectively) or 10 years (73·8, 69·2 and 69·3%, respectively) post-transplant. Patient survival was similar in all FcγRIIB-I/T232 genotype groups (Table 1).
In cohort B (African Caribbean transplant recipients) there was no significant difference in death-censored allograft survival, the frequency of treatment for rejection in the first year post-transplant or in patient survival between the individuals with different FCGR2B genotype (Table 1).
There is an increasing appreciation that the deleterious effects of alloantibody on renal transplants may occur via complement-independent pathways, as evidenced by the existence of C4d-negative AMR. Such complement-independent effects would probably be mediated via FcγRs expressed on effector cells such as neutrophils and NK cells. Of note, FcγRIIB regulates IgG-mediated activation of neutrophils, a cell type observed within the capillaries of biopsies with AMR. There is also increasing evidence that donor-specific antibodies (DSAs) activate NK cells (presumably via activating FcγRs) causing chronic allograft pathology 15,16. In this study we did not detect any statistically significant increase in early or late graft survival in individuals with the FCGR2B genotype associated with receptor dysfunction. This is in contrast to the murine data available 10, and emphasizes the limitations of mouse models and the importance of human studies to confirm the relevance of such experimental observations. It may also have implications for our understanding of the pathogenesis of chronic antibody-mediated graft damage; NK cells express only activating FcγR and do not normally express FcγRIIB. Therefore, although the de-functioning FCGR2B SNP would affect neutrophil, macrophage, dendritic and B cell activation, it would not normally influence NK cell-mediated allograft damage. Thus, our results support a role for NK cells in mediating the non-complement dependent effects of antibody on the allograft.
There are a number of caveats worth noting when interpreting our data; a failure to detect an association between FCGR2B genotype and allograft or patient survival may be due to the fact that the effect size of this SNP is smaller than estimated by our power calculations, and therefore we were under-powered to detect any differences. Larger studies with an increased number of patients may reveal such associations. In addition, the phenotypical data available on this cohort did not include detailed information on humoral alloimmune responses, such as the development of donor-specific antibody post-transplant or the presence of transplant glomerulopathy on biopsy. Therefore, we cannot exclude that the FCGR2B genotype might affect more specific aspects of antibody-mediated alloimmunity.
In summary, in two cohorts of Caucasian and African renal transplant recipients, we found no effect of FCGR2B genotype on pre-transplant PRA, acute rejection rates and graft function at 1 year, nor on 10-year transplant or patient survival.
Acknowledgments
This work was supported by a Wellcome Trust Intermediate Fellowship (WT081020) to MRC, by a grant from the Roche Organ Transplant Research Fund, and by the National Institute for Health Research Cambridge Biomedical Research Centre. K. G. C. S. was funded by a Wellcome Trust (Program Grant Number 083650/Z/07/Z).
Disclosures
M. R. C., R. M., B. D., L. W., G. O. and K. G. C. S. have no conflicts of interest to disclose.
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