New Therapies for Highly Sensitized Patients on the Waiting List

Exposure to HLA alloantigens through pregnancy, blood products, and previous transplantations induce powerful immunologic responses that create an immunologic barrier to successful transplantation. This is commonly detected through screening for HLA antibodies using Luminex beads coated with HLA antigens at transplant evaluation. Currently accepted approaches to desensitization include plasmapheresis/low-dose or high-dose intravenous Ig plus anti-CD20. However, these approaches are often unsuccessful because of the inability to remove high titer circulating HLA antibodies and limit rebound responses by long-lived anti-HLA antibody secreting plasma cells (PCs) and memory B cells (BMEM). This is especially significant for patients with a calculated panel reactive antibody of 99%–100%. Newer desensitization approaches, such as imlifidase (IgG endopeptidase), rapidly inactivate IgG molecules and create an antibody-free zone by cleaving IgG into F(ab’2) and Fc fragments, thus eliminating complement and cell-mediated injury to the graft. This represents an important advancement in desensitization. However, the efficacy of imlifidase is limited by pathogenic antibody rebound, increasing the potential for antibody-mediated rejection. Controlling antibody rebound requires new strategies that address the issues of antibody depletion and inhibition of BMEM and PC responses. This will likely require a combination of agents that effectively and rapidly deplete pathogenic antibodies and prevent immune cell activation pathways responsible for antibody rebound. Here, using anti–IL-6 receptor (tocilizumab) or anti–IL-6 (clazakizumab) could offer long-term control of BMEM and PC donor-specific HLA antibody responses. Agents aimed at eliminating long-lived PCs (anti-CD38 and anti–B-cell maturation antigen×CD3) are likely to benefit highly HLA sensitized patients. Complement inhibitors and novel agents aimed at inhibiting Fc neonatal receptor IgG recycling will be important in desensitization. Administering these agents alone or in combination will advance our ability to effectively desensitize patients and maintain durable suppression post-transplant. After many years of limited options, advanced therapeutics will likely improve efficacy of desensitization and improve access to kidney transplantation for highly HLA sensitized patients.


Introduction
HLA sensitization poses a significant and substantial immunologic barrier to life-saving organ transplantation.2][3] The US Kidney Allocation System permits priority for sensitized candidates, allowing an increased opportunity for deceased donor kidney transplantation in HLA sensitized (HS) patients. 4,5Unfortunately, transplantation rates remain low for the most highly HS candidates (calculated panel reactive antibody [cPRA] .99.9%) because of unacceptable cross-matches. 6,7In fact, patients with this level of sensitization are more likely to die or be removed from the wait list than be transplanted. 6][10] Desensitization protocols have progressed considerably and have benefited from the introduction of therapies for autoimmunity and B-cell/plasma cell (PC) malignancies which decrease deleterious IgG molecules, target critical cytokines, and eliminate B cells and PCs.Another critical advancement has been our ability to stratify immunologic risk using Luminex single antigen bead assays. 11,12This allows a window of opportunity to successfully transplant HS patients with decreased risks.In this review, we will not discuss the basic concepts and approaches to desensitization.These basic concepts are discussed in references presented here. 6,8,13,14Many new desensitization strategies have emerged to address this vexing immunologic problem (Table 1).However, there are currently no US Food and Drug Administration agents approved for desensitization.

Translating Advances in B-Cell Immunology into Effective Desensitization Therapies
Advances in B-cell immunology have been critical to understanding the antigenic anatomy of B cells, PCs, and effector elements (natural killer [NK] cells and macrophages) of antibody-mediated responses to allografts.The information gleaned from understanding the importance of anti-HLA antibodies and cellular elements that support their production and effector functions has allowed us to develop novel compositional approaches for the treatment of allosensitized patients.Critical to the first element, anti-HLA antibodies, newly developed agents such as imlifidase and Fc neonatal receptor (FcRn) inhibitors, and anti-CD38 antibodies address antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) injury, which is the critical first step in a successful desensitization protocol.
An ideal desensitization agent should reduce donorspecific HLA antibody (DSA), prevent CDC and ADCC, and most importantly, reduce deleterious antibody rebound.Antibody rebound is a well-recognized but poorly defined phenomenon characterized by rapid increases in pathogenic antibodies after removal by plasma exchange or IgG (by imlifidase).We assume that key cytokines, such as B-cell activating factor of TNF family/a proliferation inducing ligand and IL-6, stimulate B MEM cells and PCs to rapidly produce antibody after these therapies.These pathways are summarized in (Figure 1A).In addition, therapeutics aimed at decreasing antibody levels and targeting DSA effector functions, CDC, and ADCC are essential 15 (Figure 1B).
Current approaches to desensitization include plasmapheresis (PLEX) and intravenous Ig (IVIg).PLEX removes circulating antibodies (along with other plasma proteins).][18] Our group first reported on high-dose IVIg used for desensitization. 19IVIg, derived from pooled plasma of thousands of donors, has numerous effects on antibodies and effector cell functions, including blocking FcRn to limit antibody recycling, thus reducing antibody half-life, neutralizing pathogenic antibody effector functions via idiotypic or anti-idiotypic reactions, inhibiting complement activation, and activating immune inhibitory signaling through FcƴRIIb receptors on immune cells. 20IVIg is an important component of current desensitization protocols.It is also beneficial in preventing opportunistic infections in patients undergoing desensitization. 21However, the combination of IVIg1PLEX does not prevent the activation of wellestablished B MEM and donor-reactive PCs producing pathogenic antibodies.
Targeting Critical Effector Pathways ADCC and CDC ADCC and CDC are critical effector pathways mediating pathogenic antibody injury and destruction of cognate antigen-binding targets.The ability of IgG Fc receptors to recognize antibodies bound to these targets is essential to this effector function.In kidney transplantation, the targets are usually HLA antigens on donor vascular endothelium.For CDC, critical segments of the Fc region of DSAs activate C1Q and initiate the classic complement pathway to form C5b-C9 membrane attack complex (MAC), which destroys endothelial cells and results in graft injury.ADCC is mediated by NK cells, monocytes, and polymorphonuclear leukocytes.Fc-receptor binding initiates cytotoxic enzyme release (perforin and granzyme), destroying the endothelium and perpetuating graft injury.Both processes are important and can occur in concert (or separately).However, it appears that in early severe graft rejection, CDC is more critical, whereas in chronic antibody-mediated rejection (cAMR), ADCC plays a more important role.
0][31] Anti-CD20 therapeutic advancement, including obinutuzumab (type 2 anti-CD20) development, demonstrated clinical superiority to rituximab in B-cell lymphoma and systemic lupus erythematosus treatment. 32IVIg1obinutuzumab for desensitization failed to demonstrate efficacy in decreasing anti-HLA antibodies in HS patients. 33However, obinutuzumab demonstrated significant reductions of B cells, B MEM, and plasmablasts in patients with ESKD.This suggests that obinutuzumab may be important in modifying rebound antibody responses from B MEM cells and plasmablasts.It is important to note that obinutuzumab is very effective in reducing/ eliminating autoantibodies versus alloantibodies.This likely represents a targeting issue because autoantibodies are primarily generated from CD20 1 plasmablasts while HLA antibodies are produced by long-lived PCs, which are CD20 deficient.

IL-6/IL-6 Receptor Inhibition
IL-6 is a multifunctional cytokine critical for T-follicular/ helper cells (T fh ) and adaptive immune responses. 34,35A review of data investigating inhibition of IL-6/IL-6 receptor (IL-6R) signaling pathway suggests that it may provide benefits in reducing pathogenic IgG antibodies and T-effector/memory responses while increasing T REG populations, limiting endothelial cell activation and injury in response to DSA binding. 36

Tocilizumab (Anti-IL-6R)
Tocilizumab (TCZ) is a humanized anti-IL-6R mAb.Our group evaluated TCZ1IVIg desensitization in ten HS kidney transplant (KT) recipients who failed standard desensitization. 37Five patients were transplanted.No AMR was seen on 6-month protocol biopsies, and DSA reductions occurred.Daligault et al. reported on TCZ administration   APCs process and present alloantigens to naı ¨ve CD4 1 T cells.This process requires alloantigen binding to cognate TCR complexes followed by costimulation through CD80/86 (APC) and CD28 (naı ¨ve T cell).Activated CD4 1 cells then migrate to regional lymph nodes and spleen.Here, under the influence of CXCR5, they mature into T-follicular (T FH ) cells that stimulate alloantigen responses in naı ¨ve B cells stimulated by the cytokines (IL-6 and IL-21).Activated B-cells develop into B MEM cells with specific cells evolving into plasmablasts and ultimately to antibody-producing PCs.BAFF and APRIL are also critical to this process, interacting with BCMA (BCMA on PCs and some B cells).Antibody affinity to graft alloantigens (DSAs) evolve from low-affinity IgG (purple) to high affinity, complement activating alloantibodies (in red) binding to donor-specific HLA molecules in the allograft, and initiate the clinical and pathologic features of AMR.The antigenic anatomy of each cell type is shown that could represent relevant targets for therapeutic intervention.(B) The therapeutic approaches for prevention and treatment of AMR that are shown in conjunction with their targeted alloantigen activation and effector pathways.As discussed in the text, the major pathogenic factor is high-affinity IgG complement activating DSAs.Initial alloactivation events leading to T-cell activation can be blocked by CTLA4Ig or anti-CD28.Importantly, these costimulatory blockers are also potent inhibitors of germinal center (Tfh) activity and can prevent primary immune events induced by alloantigens.New data also suggest that CTLA4-Ig may directly inhibit CD80/861 PCs.Inebilizumab (anti-CD19) depletes B cells to treatment-naïve HS patients (N 5 14). 38TCZ significantly reduced dominant anti-HLA antibody sensitization.Mean fluorescence intensity (MFI) decrease was minor, with only one patient transplanted.Jouve et al. conducted the TETRA study where monthly TCZ 3 6 months, given before standard of care regimen, found TCZ1standard of care could limit post-transplant HLA antibody rebound, but reductions in pretransplant MFIs were not clinically significant. 39azakizumab (Anti-IL-6) Clazakizumab, an IgG1 anti-IL-6 mAb, 40 was evaluated for desensitization at our center.Here, 20 HS patients (cPRA .50%) received PLEX 1 IVIg and then clazakizumab monthly 3 6 months. 41If transplanted, patients received clazakizumab 3 12 months.After 6 months, clazakizumab treatment was associated with significant reductions in HLA antibodies, including strong-binding HLA antibodies (.10,000 MFI) with 18 patients transplanted.Fourteen patients (78%) were DSA positive at transplant.At 12 months, only one patient remained DSA positive, patient and graft survival were 100% and 94%, respectively, and the mean eGFR was 58 6 29 ml/min per 1.73 m 2 .Two other clazakizumab studies in patients with cAMR 42,43 showed reduced DSA levels, pathologic and molecular features of cAMR, and, importantly, eGFR stabilization.However, preliminary data analysis from a phase 3 randomized controlled trial of clazakizumab for cAMR treatment in patients undergoing KT (IMAGINE) failed to show efficacy at 1 year. 44This study is now closed.Nonetheless, anti-IL-6/IL-6R treatments are likely important in desensitization and DSA rebound prevention and still hold promise for treatment of cAMR in certain populations.

PC-Directed Therapies
Because long-lived PCs are the key producers of HLA antibodies, it seems reasonable to develop PC-directed therapies.

Proteosome Inhibitors Bortezomib
Bortezomib, a proteosome inhibitor used to treat multiple myeloma (MM), was examined for AMR treatment.Unfortunately, data from a well-conducted, placebo-controlled trial showed no significant capability to reduce DSAs, stabilize kidney function, or affect patient/graft survival. 45esensitization studies showed no meaningful HLA antibody level declines. 46In addition, studies showed substantial adverse event/serious adverse events with bortezomib administration.

Carfilzomib
Carfilzomib, a second-generation proteosome inhibitor, may be more robust in depleting PCs with less neuropathic side effects.Carfilzomib 1 PLEX were assessed for desensitization in 13 HS candidates undergoing KT. 47 MFI was modestly reduced in most, but antibodies rebounded to baseline levels within 5 months.A study of carfilzomib in patients undergoing lung transplant with AMR appeared to decrease DSAs and improve lung function. 48One-year graft survival for patients responsive to carfilzomib treatment was 78% versus 20% for nonresponders.Another study in lung transplant recipients showed similar results. 49No deaths occurred during treatment, but at 1 year, seven of 14 patients had died from graft failure.Although promising, long-term therapy with agents preventing DSA rebound and allograft injury are likely required to sustain initial benefits seen with carfilzomib.

Daratumumab
Daratumumab is a humanized IgG1k mAb with specificity for CD38 transmembrane glycoproteins on PCs, plasmablast, NK, and T REG cell surfaces. 50In a primate model of allosensitization, daratumumab reduced DSAs and slightly improved kidney allograft survival, 51 but a rapid antibody rebound with intense T cell-mediated rejection (CMR) was seen at 1 month, suggesting short-lived PC depletion.Jordan et al. reported using daratumumab for desensitization in a HS heart transplant candidate and resistant AMR treatment in a KT recipient. 52HLA antibodies decreased in both patients.AMR findings improved in the patients undergoing KT after daratumumab, but CMR developed on subsequent biopsy.This was also reported by others. 53,54Vo et al. reported daratumumab use for desensitization in ten HS patients, who failed several previous desensitization treatments before receiving daratumumab 6 PLEX treatment. 55Daratumumab reduced DSA MFI strength and enabled HLA incompatible transplantation in eight of ten patients.At 12 months, patient/graft survival was 100%/100% and eGFR was 73622 ml/min per 1.73 m 2 .Daratumumab was well tolerated without significant SAEs.

Isatuximab
Isatuximab is an anti-CD38 approved for treatment of MM.Vincenti et al. reported on a clinical trial of isatuximab for desensitization.Overall, cPRA values were minimally affected, with only 9 of 23 patients (39%) having cPRA reductions to target levels.By study cutoff, six patients received transplant offers, of which four were accepted.Isatuximab was considered safe with some effect on PRA reduction.

Costimulatory Blockade and Combination Therapy
Costimulatory blockade primarily inhibits naïve CD4 1 T-cell activation by impeding CD28/B7-1/B7-2 costimulation. 57,58he prototype fusion protein (cytotoxic T-lymphocyte antigen 4-Ig [CTLA4-Ig]) interferes with signal 2. Data from in vitro models suggest that naïve B cells activated by T FH cells in germinal centers are inhibited by CTLA4-Ig. 58This likely explains the benefits of CTLA4-Ig on limiting de novo DSA development.CTLA4-Ig inhibits IgG production by PCs in animal models but is not validated in humans. 58Importantly, CTLA4-Ig does not inhibit CD8 1 T-effector/T-memory which contributes to the high rejection rates seen in patients receiving CTLA4-Ig without calcineurin inhibitor immediately post-transplant.Recent studies in a primate model of KT demonstrated the inability of CTLA4-Ig to modify CD8 1 T-effector/memory cells in animals maintained on CTLA4-Ig alone.However, this was ameliorated with TCZ coadministration 59 with significant reductions in CD8 1 cell populations.
Chandran et al. reported preliminary data from a phase 1/ 2 trial evaluating daratumumab followed by belatacept as desensitization in cPRA $99.9% patients, 60 with one patient transplanted.Jackson et al. also reported a pilot study of carfilzomib with belatacept for desensitization in HS patients. 61Two patients met the efficacy end point (eliminating one HLA antibody or $50% MFI reduction of $3 HLA antibodies).Although encouraging, more data are needed. 61ain et al. assessed the efficacy of bortezomib1CTLA4-Ig in six patients undergoing KT as rescue therapy for acute AMR or mixed rejection. 62DSA (class 1 and 2) reductions were seen within 25 days of treatment.DSA levels remained low or undetectable for a 10-to 30-month follow-up.

Imlifidase
4][65] Imlifidase cleaves IgG antibodies at the IgG hinge region, resulting in two fragments, F(ab')2 and Fc.This enzymatic separation of the intact IgG components disrupts IgG-mediated ADCC and CDC.Imlifidase cleaves the B-cell receptor from B cells and selectively cleaves IgG antibodies without affecting other immunoglobulin molecules.This allows the beneficial immune responses rendered by other antibody types to remain.
Desensitization studies using imlifidase are promising.In a phase 1/2 trial, imlifidase given to HS patients before transplantation (N 5 25) completely eliminated HLA antibodies with 24 patients transplanted. 65Data from this and other studies resulted in conditional approval of imlifidase by the European Medicines Agency for desensitization of cross-match positive individuals awaiting deceased donor kidney transplantation. 66Imlifidase is being evaluated in a phase 3 trial (ConfIdeS) for desensitization in HS (.99.90%) patients undergoing KT (NCT05369975). 67ignificant issues limit imlifidase use.Importantly, it can only be used once because of immunogenicity resulting in anti-imlifidase neutralizing antibody formation 68 ; this has encouraged attempts to develop a less immunogenic imlifidase.In addition, imlifidase needs to be accompanied by other treatments to reduce rebound IgG production.

FcRn Inhibitors
Another potential approach to desensitization uses novel agents to inhibit FcRn recycling, which decreases pathogenic IgG half-life.Here, identification of FcRn recycling of IgG to extend IgG half-life (approximately 28 days) was an important breakthrough.Notably in FcRn (2/2) animals, IgG halflife is only 3 days.In depth reviews of FcRn potential applications in transplantation are available. 69,70Knechtle et al. assessed the efficacy of anti-FcRn monoclonals administered before and after kidney transplantation in rhesus macaque monkeys, sensitized by skin grafting. 71The authors found perioperative anti-FcRn mAb administration interfered with total circulating IgG via FcRn-mediated recycling, without reducing levels, likely because of ongoing DSA produced by PCs.The authors concluded that anti-FcRn treatment transiently reduced DSAs without affecting DSA synthesis.
The first US Food and Drug Administration-approved FcRn inhibitor, efgartigimod, is an Fc fragment with high affinity for the FcRn, resulting in reduced circulating IgG half-life.Efgartigimod was approved for the treatment of myasthenia gravis. 72Clinical trials are being planned for treatment of AMR. Figure 2 summarizes the effector pathways of CDC and ADCC and their importance in mediating injury to allograft endothelial cells.This demonstrates how therapies aimed at inhibiting CDC/ADCC are critical to prevent graft endothelium injury. 73specific Antibody Targeting B-Cell Maturation Antigen and CD3 REGN5459/5458 are anti-B-cell maturation antigen (BCMA)3anti-CD3 bispecific antibodies, binding BCMA on PCs and CD3 on cytotoxic T lymphocytes.REGN5459/ 5458 brings CD3 1 cells in proximity to PCs with subsequent PC killing by the activated CD3 1 cell.A phase 1/2, open-label study of REGN5459 (or REGN5458) to desensitize HS patients with ESKD is ongoing. 74REGN5459/5458 may offer benefits over anti-CD38 which targets multiple cell types because BCMA is primarily located on PCs.REGN5459/ 5458 appears to be effective in early MM clinical trials. 75ere, it is important to discern the antigenic anatomy of PCs responsible for HLA DSA generation.If they specifically express BCMA, this approach could be promising.

Chimeric Antigen Receptor-T Cells (CD19) and Chimeric Antigen Receptor-T Cells (BCMA)
Studies are underway to examine chimeric antigen receptor (CAR) T cells (CAR-T; CD19 and BCMA) as potential desensitization agents (NCT06056102).Currently, there is little information on this approach; however, Jackson et al. reported a retrospective analysis of HLA antibodies in HS patients receiving CAR-T (CD19) treatment for MM. 76nterestingly, the investigators saw no meaningful reductions in HLA antibodies after CAR-T (CD19) treatment.CAR-T (BCMA) impact on HLA antibodies was not analyzed.Investigators concluded that HLA antibody producing cells are likely CD19 NEG .CD19 is expressed on all B cells and approximately 50% of PCs.This may be likely the experience with obinutuzumab (anti-CD20), which depletes plasmablasts and some PCs, without affecting HLA antibodies in HS patients. 32,77It is important to determine whether there are meaningful differences in efficacy of CAR-T cells compared with bispecific antibodies which mimic CAR-T cells actions.

Complement Inhibition Eculizumab (Anti-C5)
Eculizumab is a mAb aimed at complement activation initiated by C5 convertase resulting in formation of the MAC (C5b-C9MAC).Treatment results in terminal complement inhibitions by preventing (C5b-C9MAC) formation.Stegall et al. showed that eculizumab significantly reduced AMR incidence compared with a historical control of patients desensitized with PLEX 1 IVIg (7.7% (2/26) versus 41.2% (21/51; P 5 0.003). 78Although the results were encouraging, patients receiving long-term eculizumab with DSA positivity did not show a difference in cAMR rates versus controls.This is an important demonstration of the need for antibody reduction therapies combined with CDC inhibition for good outcomes.Without DSA depletion, ADCC is not mitigated and results in cAMR.
Lefaucheur et al. reported on 931 KT recipients who had determinations of complement activating DSAs, biopsy assessments, and molecular scores. 79The investigators reported that eculizumab significantly improved graft survival and was specifically effective against rejection phenotypes associated with complement activating DSAs.Assessment of C1Q activating DSAs is not done in most US centers.Here, one can usually assume that DSAs of .10,000MFI are complement activating. 80mportantly, eculizumab use for desensitization needs careful consideration.First, it increases expense.Second, eculizumab has a very short half-life because it is not Fcmodified to enhance recycling through the FcRn system.There are novel strategies to improve the half-life of eculizumab through Fc engineering to enhance Fc/FcRn interactions at pH 6.5 and demonstrate an improved antibody half-life and therapeutic efficacy. 81,82Ravulizumab, an Fc-engineered anti-C5, developed from eculizumab, has enhanced binding to the FcRn.IgG-Fc/FcRn interactions prolong therapeutic antibody half-life.In addition, alterations in F(ab) amino acid structure allows C5 release at pH 6.5 when anti-C5 is bound to FcRn, allowing ligand (C5) degradation in endosomes.This allows refreshed anti-C5 to be released into the circulation, allowing continuous C5 shuttling to the endosomes for degradation.In summary, anti-C5 remains a valuable therapy for AMR where clear evidence of rapid graft decline is associated with complement (C4d) deposition and the presence of complement-activating anti-HLA antibodies.

C1 Esterase Inhibitor (C1-INH)
C1-INH prevents complement activation via the classical and MBL pathways.C1-INH has been investigated as an adjunct to desensitization.Data from animal models 83,84 and a human trial by Jordan et al. suggest that C1-INH treatment ameliorates ischemia/reperfusion injury. 85,86ere, C1-INH's prevention of ischemia/reperfusion injury likely prevents immune activation events within the allograft, leading to B-cell activation and alloantibody production. 87Vo et al. also investigated C1-INH for AMR prevention in HS KT recipients, with acceptable safety outcomes. 88C1-INH treatment significantly reduced C1q 1 HLA antibodies.In addition, post-transplant, C1-INH treated patients had lower DSA rebound rates and de novo DSA development versus placebo.This is consistent with the C1q-initiated complement activation that drives antibody affinity maturation. 89n a study evaluating a humanized mAb targeting C1s, investigators showed inhibition of alloantibody initiated classical pathway complement activation in vitro. 90he authors performed a trial of anti-C1s in AMR patients (NCT02502903). 91There were no severe AEs, and anti-C1s completely eliminated DSA-triggered classic pathway activation.In addition, C4d 1 staining was significantly reduced in most patients.Despite this, there were no meaningful reductions in microcirculation inflammation, gene expression patterns, DSA levels, or kidney function improvement.Again, selective CDC inhibition, without DSA reductions, has limited benefits (Table 1).

Conclusions
Currently, there is a rapid emergence of new agents aimed at desensitization and AMR treatment.Multiple agents with varying mechanisms of action include imlifidase and FcRn inhibitors, anti-cytokine (IL-6/IL-6R) therapies, and PCdirected therapies.However, no single agent can control all facets of antibody generation and injury.The future of antibody-directed therapeutics will require protocols consisting of several agents placed in a logical sequence to rapidly remove pathogenic DSAs and prevent their reemergence.

Disclosures
Disclosure forms, as provided by each author, are available with the online version of the article at http://links.lww.com/KN9/A572.

Funding
None.

Figure 1 .
Figure 1.Key cytokines for immune activation.(A) The immune activation pathways critical for development of alloimmunity.APCs process and present alloantigens to naı ¨ve CD41 T cells.This process requires alloantigen binding to cognate TCR complexes followed by costimulation through CD80/86 (APC) and CD28 (naı ¨ve T cell).Activated CD4 1 cells then migrate to regional lymph nodes and spleen.Here, under the influence of CXCR5, they mature into T-follicular (T FH ) cells that stimulate alloantigen responses in naı ¨ve B cells stimulated by the cytokines (IL-6 and IL-21).Activated B-cells develop into B MEM cells with specific cells evolving into plasmablasts and ultimately to antibody-producing PCs.BAFF and APRIL are also critical to this process, interacting with BCMA (BCMA on PCs and some B cells).Antibody affinity to graft alloantigens (DSAs) evolve from low-affinity IgG (purple) to high affinity, complement activating alloantibodies (in red) binding to donor-specific HLA molecules in the allograft, and initiate the clinical and pathologic features of AMR.The antigenic anatomy of each cell type is shown that could represent relevant targets for therapeutic intervention.(B) The therapeutic approaches for prevention and treatment of AMR that are shown in conjunction with their targeted alloantigen activation and effector pathways.As discussed in the text, the major pathogenic factor is high-affinity IgG complement activating DSAs.Initial alloactivation events leading to T-cell activation can be blocked by CTLA4Ig or anti-CD28.Importantly, these costimulatory blockers are also potent inhibitors of germinal center (Tfh) activity and can prevent primary immune events induced by alloantigens.New data also suggest that CTLA4-Ig may directly inhibit CD80/861 PCs.Inebilizumab (anti-CD19) depletes B cells

Figure 2 .
Figure 2.This figure demonstrates the importance of ADCC and CDC as critical antibody effector pathways responsible for endothelial cell injury and ultimately, antibody rejection.First, ADCC is mediated by NK cells, monocytes, and PMNs through recognition of Fc fragments of pathogenic DSAs bound to endothelial cell targets (HLA molecules).Interaction of FcƴR1 cells with Fc of target bound DSAs initiates cell activation with the release of perforins and granzyme, destroying the target cell.NK cells also release chemokines and cytokines that result in recruitment of monocytes and macrophages that increase ADCC and cell adhesion to the target endothelium.CDC is mediated primarily by activation of the classic complement pathway by pathogenic DSAs.This activation ultimately results in formation of the C5b-C9 MAC which destroys target cells.During complement activation, potent anaphylatoxins (C3a and C5a) are released and are critical for initiation of monocyte and NK cell recruitment.In addition (not shown) C3a and C5a bind to receptors on B cells and T cells resulting in activation of T-cell-directed immune responses and enhancement of high-affinity DSA production.Important therapeutic agents to combat ADCC and CDC include imlifidase which removes Fc from DSAs and thus inhibits their ability to mediate these important injurious pathways.This also likely true for the new FcRn inhibitors which rapidly reduce circulating IgG (DSA) by inhibiting IgG recycling mechanisms.This would also be true for high dose IVIg.In addition, anticomplement therapies (anti-C5 and C1-INH) inhibit classic, mannose binding and alternative pathways, limiting CDC and activation of immune cells.ADCC can also be inhibited by use of anti-CD38 which can deplete NK cells and monocytes/macrophages as shown.(Modified from: Peerview.com/RBD.)MAC, membrane attack complex; NK, natural killer; PMN, polymorphonuclear leukocyte.

Table 1 .
(Continued)New Therapies for Highly Sensitized Patients, Vo et al.