Antigen-guided Depletion of Anti-HLA Antibody-producing Cells by HLA-Fc Fusion Proteins
Webber AM, et al. Blood 2022;140:1803–1815.
HLA molecules are highly diverse and the main target of donor-specific antibodies (DSAs). The presence of alloreactive antibodies against HLA molecules is a major barrier for transplantation. Clinically, the development of DSAs against donor HLAs following transplantation is associated with shortened graft survival. Several treatments targeting such antibodies, including plasmapheresis, IVIG, anti-CD20, proteasome inhibitors, and IdeS, have been evaluated clinically with some promises. However, these therapies deplete antibodies, B cells‚ or plasma cells in a nondirected manner, often leading to overimmunosuppression and associated comorbidities.
Webber and coworkers present a new class of therapeutics—HLA-Fc fusion proteins.1 The HLA-Fc fusion protein can act as a decoy for circulating anti-HLA antibodies to bind, thus diverting them from their targets on donor organs. But perhaps more importantly, HLA-Fc fusion proteins bind to HLA-specific surface immunoglobin on memory B cells, exerting Fc-mediated complement-dependent cytotoxicity upon engagement. In a proof-of-concept study, the authors designed 2 HLA-Fc fusion proteins by combining the Fc from mouse IgG2a and A2 single-chain trimer or B7 single-chain trimer. When added to sera of HLA-A2 or B7 sensitized transplant patients, both fusion proteins can neutralize respective anti-HLA antibodies with high specificity. Additionally, the authors tested both fusion proteins on hybridoma cells derived from mouse B cells with anti-A2 and -B7 specificities. They found that A2-Fc and B7-Fc bind to hybridoma cells of cognate specificity only, and upon binding can initiate downstream complement-dependent antigen-specific killing in vitro. The Fc region is essential for this observed action as a mutated Fc-silenced version of A2-Fc (A2Fc-LALAPG) only binds to hybridoma cells but demonstrates no cytotoxicity. Finally, the authors tested A2-Fc in vivo using an immunodeficient mouse model infused with hybridoma cells with anti-A2 specificity. In those animals, hybridoma cells secrete anti-A2 antibody into the circulation; without intervention, animals would succumb to overgrowth of hybridoma cells. However, infusion of A2-Fc, but not its Fc-silenced counterpart, effectively depleted the hybridoma cells. Like anti–A2-specific hybridoma cells, A2 positive human platelets were also protected by the coadministered A2-Fc by prevention of anti–A2-mediated platelet destruction.
Overall, this study presents a novel class of fusion proteins that target anti-HLA antibodies as well as anti-HLA antibody-producing cells. There are several advantages of HLA-Fc fusion proteins compared with existing therapies: (1) HLA-Fc fusion proteins are dual targeting, simultaneously neutralizing anti-HLA antibodies while depleting antibody-producing memory B cells; (2) the approach is versatile and can be designed to target antibodies and B cells reactive to specific HLA molecules; and (3) HLA-Fc fusion proteins are by design specific, thus sparing protective global humoral immunity. Thus, HLA-Fc fusion proteins represent yet another example of precision medicine of individualized approaches rather than broadly acting agents.
Interestingly, the authors noticed A2-Fc with a nonfunctional Fc stimulated modest production of anti-A2 antibody‚ whereas such a phenomenon was not noted with A2-Fc with a functional Fc. The authors suggest that a functional Fc may have inhibited alloimmunization to A2-Fc. However, it is well established that the host can produce anti-drug antibodies against an array of biologics, many of which are antibodies with a functional Fc domain.2 Although additional sensitization to the HLA molecule was not observed in animals treated with A2-Fc, this observation raises concerns of sensitizing the host rather than depleting cells producing anti-HLA antibodies. It is also important to recognize that HLA-Fc fusion proteins are not anticipated to target DSA-producing plasma cells due to the lack of surface immunoglobulin on IgG producing plasma cells. However, these cells are thought to be the primary source of antibody rebound following conventional desensitization therapies. Finally, the HLA-Fc fusion proteins were only tested in vitro or in vivo in an immunodeficient host. Therefore, the effects of HLA-Fc fusion proteins on the rest of the immune system, particularly the T lymphocytes, cannot be determined. Future studies will therefore need to evaluate the effect of HLA-Fc fusion proteins in immunocompetent hosts and in clinically relevant models.
The Gut Microbe Bacteriodes fragilis Ameliorates Renal Fibrosis in Mice
Zhou W, et al. Nat Commun 2022;13:6081.
Chronic kidney disease (CKD) of various causes inevitably progresses to renal fibrosis characterized by the proliferation of fibroblasts and myofibrolasts. Management of CKD is currently limited to conservative measures such as optimizing the control of blood pressure, diabetes, proteinuria, and limiting dietary salt intake.1 Established therapeutics targeting and slowing renal fibrosis and development of end-stage kidney disease are currently not available.
Zhou and coworkers2 examined and established a causative relationship between the gut microbiota Bacteriodes fragilis (B. fragilis) and the progression of renal fibrosis. First, in 2 independent cohorts of CKD patients in China, the authors established that fecal B. fragilis correlated negatively with the stage of CKD assessed by serum BUN and creatinine. Next, the authors used 2 CKD murine models, namely the unilateral ureteral obstruction and adenine-induced CKD, to dissect the mechanisms of B. fragilis–mediated renal protection and prevention of renal fibrosis. Phenotypically, they observed that oral administration of live B. fragilis decreased serum level of LPS, inhibited inflammation‚ and attenuated renal fibrosis. Mechanistically, they demonstrated that B. fragilis administration led to an increase of sodium-glucose cotransporter-2 (SGLT2) in renal epithelial cells, which in turn led to an increased renal reabsorption of 1,5-anhydroglucitol (1,5-AG), a 1-deoxy form of glucose. Using an in silico method, the authors determined that 1,5-AG was an agonist of the transmembrane G protein–coupled bile acid receptor (TGR5); activation of TGR5 in vitro resulted in an inhibition of oxidative stress, reduced TGF-β/Smad signaling and decreased expression of profibrotic genes. Conversely, knockdown of TGR5 in vitro abolished the antifibrotic effect of 1,5-AG. The authors further determined that patients with CKD had significantly decreased serum levels of 1,5-AG in 2 independent patient cohorts. Interestingly, they also found that a natural compound called madecassoside from herbal medicines could promote gut B. fragilis growth. Consequently, oral but not intraperitoneal, administration of madecassoside ameliorated renal fibrosis in the unilateral ureteral obstruction mice; this effect was abolished if gut microbiota were eliminated by a cocktail of antibiotics.
This study provides an intriguing piece of evidence that gut microbiota, specifically B. fragilis, play a key role in the development and progression of CKD. More importantly, several mechanistic steps through which B. fragilis exerts its protective effect against CKD, namely, SGLT2, 1,5-AG, and TGR5, as well as an unexpected revelation of SGLT2 as a 1,5-AG transporter in the kidney are of relevance. As such, the gut microbiota provide several potential therapeutic targets that can be exploited to retard CKD progression. Upstream, B. fragilis containing probiotics or B. fragilis growth-promoting agents may slow CKD progression. Indeed, the authors showed that madecassoside and the dietary byproduct 1,5-AG are agents with that potential. Downstream, targeting TGR5 using small molecule inhibitors can also be exploited in the future as a potential CKD treatment, although in the current study only in vitro genetic knockdown of TGR5 was experimented. SGLT2 targeting may be trickier, as inhibitors of SGLT2 have been shown to be beneficial in controlling type 2 diabetes, along with several other cardioprotective and renoprotective effects.3 In the current study, empagliflozin, an SGLT2 inhibitor, significantly decreased serum concentration of 1,5-AG in mice. Therefore, the role of SGLT2 in different pathways slowing CKD progression warrants further clarification.
In summary, the current study demonstrates that gut microbiome could attenuate renal fibrosis and delay CKD progression.
1. Webber AM, Bradstreet TR, Wang X, et al. Antigen-guided depletion of anti-HLA antibody–producing cells by HLA-Fc fusion proteins. Blood. 2022;140:1803–1815.
2. Krishna M, Nadler SG, Nadler SG. Immunogenicity to biotherapeutics - the role of anti-drug immune complexes. Front Immunol. 2016;7:21.
1. Turner JM, Bauer C, Abramowitz MK, et al. Treatment of chronic kidney disease. Kidney Int. 2012;81:351–362.
2. Zhou W, Wu WH, Si ZL, et al. The gut microbe Bacteroides fragilis ameliorates renal fibrosis in mice. Nat Commun. 2022;13:6081.
3. McMurray JJV, Wheeler DC, Stefansson BV, et al.DAPA-CKD Trial Committees and Investigators. Effect of dapagliflozin on clinical outcomes in patients with chronic kidney disease, with and without cardiovascular disease. Circulation. 2021;143:438–448.