IGG3 anti-HLA donor-specific antibodies and graft function in pediatric kidney transplant recipients

The role of antibody glycosylation in autoimmune and alloimmune kidney diseases

Immunoglobulin glycosylation is a pivotal mechanism that drives the diversification of antibody functions. The composition of the IgG glycome is influenced by environmental factors, genetic traits and inflammatory contexts. Differential IgG glycosylation has been shown to intricately modulate IgG effector functions and has a role in the initiation and progression of various diseases. Analysis of IgG glycosylation is therefore a promising tool for predicting disease severity. Several autoimmune and alloimmune disorders, including critical and potentially life-threatening conditions such as systemic lupus erythematosus, anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis and antibody-mediated kidney graft rejection, are driven by immunoglobulin. In certain IgG-driven kidney diseases, including primary membranous nephropathy, IgA nephropathy and lupus nephritis, particular glycome characteristics can enhance in situ complement activation and the recruitment of innate immune cells, resulting in more severe kidney damage. Hypofucosylation, hypogalactosylation and hyposialylation are the most common IgG glycosylation traits identified in these diseases. Modulating IgG glycosylation could therefore be a promising therapeutic strategy for regulating the immune mechanisms that underlie IgG-driven kidney diseases and potentially reduce the burden of immunosuppressive drugs in affected patients.

Analysis of Cross-sectional and Longitudinal HLA and Anti-viral Responses After COVID Infection in Renal Allograft Recipients: Differences and Correlates

BACKGROUND

Characterization of anti-HLA versus anti-severe acute respiratory syndrome coronavirus 2 (anti-SARS-CoV-2) immune globulin isotypes in organ transplant recipients after coronavirus disease 2019 (COVID-19) infection has not been reported. We aimed to determine changes in anti-HLA antibodies in renal transplant patients with COVID-19 and compare the immunoglobulin and epitope-binding pattern versus anti-SARS-CoV-2 antibodies.

METHODS

This is a cross-sectional study of 46 kidney transplant recipients including 21 with longitudinal sampling. Using a semi-quantitative multiplex assay, we determined immunoglobulin (Ig) M, IgA, IgG, and IgG1-2-3-4 antibodies against Class I and Class II HLA, and 5 SARS-CoV-2 epitopes including the nucleocapsid protein and multiple regions of the spike protein.

RESULTS

Fourteen of 46 (30%) patients had donor-specific anti-HLA antibodies (donor-specific antibody [DSA]), 12 (26%) had non-DSA anti-HLA antibodies and 45 (98%) had anti-SARS-CoV-2 antibodies. Most DSAs targeted HLA-DQ (71%), with a dominant IgG isotype and IgG1 subtype prevalence (93%), and/or IgG3 (64%), followed by IgG2 (36%). Comparatively, there was a higher prevalence of IgA (85% versus 14%, P = 0.0001) and IgM (87%, versus 36%, P = 0.001) in the anti-SARS-CoV-2 antibody profile, when compared to DSAs, respectively. Anti-SARS-CoV-2 antibody profile was characterized by increased prevalence of IgM and IgA, when compared to DSAs. The median calculated panel reactive antibody before COVID-19 diagnosis (24%) tended to decrease after COVID-19 diagnosis (10%) but it was not statistically significant ( P = 0.1).

CONCLUSIONS

Anti-HLA antibody strength and calculated panel reactive antibody in kidney transplant recipients after COVID-19 do not significantly increase after infection. Although the IgG isotype was the dominant form in both HLA and SARS-CoV-2 antigens, the alloimmune response had a low IgA pattern, whereas anti-SARS-CoV-2 antibodies were high IgA/IgM.

Delayed Kinetics of IgG, but Not IgA, Antispike Antibodies in Transplant Recipients following SARS-CoV-2 Infection

BACKGROUND

Kidney transplant recipients are at increased risk of severe outcomes during COVID-19. Antibodies against the virus are thought to offer protection, but a thorough characterization of anti-SARS-CoV-2 immune globulin isotypes in kidney transplant recipients following SARS-CoV-2 infection has not been reported.

METHODS

We performed a cross-sectional study of 49 kidney transplant recipients and 42 immunocompetent controls at early (≤14 days) or late (>14 days) time points after documented SARS-CoV-2 infection. Using a validated semiquantitative Luminex-based multiplex assay, we determined the abundances of IgM, IgG, IgG1-4, and IgA antibodies against five distinct viral epitopes.

RESULTS

Kidney transplant recipients showed lower levels of total IgG antitrimeric spike (S), S1, S2, and receptor binding domain (RBD) but not nucleocapsid (NC) at early versus late time points after SARS-CoV-2 infection. Early levels of IgG antispike protein epitopes were also lower than in immunocompetent controls. Anti-SARS-CoV-2 antibodies were predominantly IgG1 and IgG3, with modest class switching to IgG2 or IgG4 in either cohort. Later levels of IgG antispike, S1, S2, RBD, and NC did not significantly differ between cohorts. There was no significant difference in the kinetics of either IgM or IgA antispike, S1, RBD, or S2 on the basis of timing after diagnosis or transplant status.

CONCLUSIONS

Kidney transplant recipients mount early anti-SARS-CoV-2 IgA and IgM responses, whereas IgG responses are delayed compared with immunocompetent individuals. These findings might explain the poor outcomes in transplant recipients with COVID-19.

PODCAST

This article contains a podcast at https://www.asn-online.org/media/podcast/JASN/2021_11_23_briggsgriffin112321.mp3.

Approaches for the characterization of clinically relevant pre-transplant human leucocyte antigen (HLA) antibodies in solid organ transplant patients

The avoidance of antibody-mediated rejection (AMR) attributed to human leucocyte antigen (HLA) antibody incompatibility remains an essential function of clinical Histocompatibility and Immunogenetics (H&I) laboratories who are supporting solid organ transplantation. Developments in HLA antibody identification assays over the past thirty years have greatly reduced unexpected positive cellular crossmatches and improved solid organ transplant outcomes. For sensitized patients, the decision to register unacceptable HLA antigen mismatches is often heavily influenced by results from solid phase antibody assays, particularly the Luminex^® Single Antigen Bead (SAB) assays, although the clinical relevance of antibodies identified solely by these assays remains unclear. As such, the identification of non-clinically relevant antibodies may proportionally increase the number of unacceptable transplant mismatches registered, with an associated increase in waiting time for a compatible organ. We reflect on the clinical relevance of antibodies identified solely by the Luminex SAB^® assays and consider whether the application of additional assays and/or tools could further develop our ability to define the clinical relevance of antibodies identified in patient sera. Improvements in this area would assist equity of access to a compatible transplant for highly sensitized patients awaiting a solid organ transplant.

Investigating the relationship between class I HLA-specific immunoglobulin-G subclasses, Pan-IgG single antigen bead assays and complement mediated interference in sera from renal transplant recipients

INTRODUCTION

Antibody mediated rejection is the leading cause of kidney transplant failure. Not all antibodies are harmful and some may be protective. Immunoglulin Gs, of which there are four subtypes, are detected by single antigen bead testing. The aims of this study were to characterise the IgG subclass profiles for class I HLA-specific antibodies in an uncensored post-transplant population and to determine the underlying relationship between reactivity patterns and MFI cut-offs with the pan-IgG assay.

METHODS

Patients were recruited to the study who were transplanted in our centre between 2009 and 2014. Prospectively stored post-transplant serum initially underwent a Labscreen Mixed assay and those positive for class I HLA-specific antibody underwent standard SAB testing, EDTA, 1 in 10 dilution and IgG subclass modifications using the Luminex platform. A total of 4947 bead reactions from 51 patients were analysed.

RESULTS

A 1 in 10 dilution was used as a comparator pan-IgG assay for summed subclass and individual subclass linear regression analyses. Using a dilution to standard assay ratio we characterised all reactions for prozone potential i.e. how likely there is to be inhibition related to complement complex formation. We stratified samples into degrees of association and were able to determine suggested MFI thresholds of Log 5.35 for the dilution assay and Log 5.05 for the summed subclass assay when considering a Log MFI of 6.9 (1000) in the standard assay. Using individual subclass dominant reactions (>70%) we were able to determine linear relationships between the 1 in 10 dilution pan-IgG assay and the individual subclass assays (excluding prozone potential reactions for IgG1/3) enabling us to suggest Log MFI thresholds of 5.03, 3.58, 4.3 and 4.05 respectively for IgG1-4.

DISCUSSION

We recommend a 1 in 10 dilution as the optimum pan-IgG comparator assay for a subclass analysis. We advocate the utilisation of the summed subclass assay to determine overall relationships and potential subclass failures. Following others, we recommend serum pre-treatment of the subclass assays to mitigate prozone. We suggest cut-offs for each IgG subclass which should be used with caution given the many inhibitory influences which may include competitive inhibition for bead binding, IgM and IgA interference and under-representation of specific subclasses on the bead panel.

Validation and cross-reactivity pattern assessment of monoclonal antibodies used for the screening of donor-specific IgG antibody subclasses in transplant recipients

The screening for IgG subclass donor-specific antibodies (DSAs) in allograft recipients uses IgG1-4 subclass-specific monoclonal antibodies (mAbs) that should be mono-specific. The cross-reactivity discrepancies reported for IgG subclass-specific mAbs warranted a critical cross-reactivity pattern analysis of the IgG subclass-specific mAbs most commonly used to detect DSAs. We tested the reactivity of 2 anti-IgG1-, 3 anti-IgG2-, 1 anti-IgG3-, and 2 anti-IgG4-specific PE-conjugated mAbs against microbeads coated with IgG1-4 proteins separately. Each IgG subclass protein was coated at three densities on the beads (0.5, 1, and 2 μg of protein per 10⁶ beads), and the PE-conjugated mAbs were titrated from 0.04 μg/mL to 5 μg/mL. The IgG subclass reactivity of the sample was acquired on the Luminex multiplex platform. Among the IgG subclass-specific mAbs, only the anti-IgG3 (clone: HP6050) mAb was mono-specific. All other mAbs tested were binding to IgG subclass proteins other than their respective immunogen, thereby being cross-reactive. IgG subclass cross-reactivity patterns were dependent on the concentration of both IgG subclass-specific mAbs and IgG1-4 protein targets coated onto the beads. With the current IgG subclass mAbs available, 3 of the 15 possible combinations of IgG1-4 subclass protein could be identified. While the remaining 12 unique combinations cannot be distinguished clearly, 6 groups that corresponded to two different unique combinations of IgG1-4 subclass protein could be identified. The dilution of serum samples and IgG subclass-specific mAbs, other than the anti-IgG3 (clone: HP6050), must be further optimized before their implementation in IgG subclass DSA screening in allograft recipients.

Distribution of de novo Donor-Specific Antibody Subclasses Quantified by Mass Spectrometry: High IgG3 Proportion Is Associated With Antibody-Mediated Rejection Occurrence and Severity

Donor-specific antibodies (DSAs) are the main risk factor for antibody-mediated rejection (ABMR) and graft loss but could have variable pathogenicity according to their IgG subclass composition. Luminex-based test might lack sensitivity for the detection of IgG subclasses and this test does not allow quantifying the relative abundance of each IgG subclass. We investigated the precise repartition of each DSA subclass and their role in ABMR occurrence and severity, using an innovative mass spectrometry-based method. Between 2014 and 2018, we enrolled 69 patients who developed de novo DSA (n = 29 without ABMR, and n = 40 with ABMR) in two transplant centers. All IgG subclasses were detected in every samples tested: 62.7% were IgG1, 26.6% were IgG2, 6.6% were IgG3, and 4.2% were IgG4. The IgG3 proportion was significantly higher in the ABMR+ compared to the ABMR– group (8.4% vs. 5.6%, p = 0.003). The proportion of IgG1, IgG2, and IgG4 of DSA was similar between the two groups. Higher IgG3 level was associated with higher C4d deposition, higher microvascular inflammation scores, and glomerular filtration rate decline >25%. IgG3 proportion was not correlated with DSA MFI. Multivariate analysis showed that proteinuria and high level of IgG3 DSA were the only two factors independently associated with ABMR. In conclusion, de novo DSA are always composed of the four IgG subclasses, but in different proportions. High IgG3 proportion is associated with ABMR occurrence and severity and with poorer outcome, independently of DSA MFI.

Inverse Association Between the Quantity of Human Peripheral Blood CXCR5+IFN-γ+CD8+ T Cells With De Novo DSA Production in the First Year After Kidney Transplant

BACKGROUND

We recently reported that a novel CXCR5IFN-γCD8 T-cell subset significantly inhibits posttransplant alloantibody production in a murine transplant model. These findings prompted the current study to investigate the association of human CD8 T cells with the same phenotype with the development of de novo donor-specific antibody (DSA) after kidney transplantation.

METHODS

In the current studies, we prospectively and serially analyzed peripheral blood CD8 and CD4 T-cell subsets and monitored for the development of de novo DSA in kidney transplant recipients during the first-year posttransplant. We report results on 95 first-time human kidney transplant recipients with 1-year follow-up.

RESULTS

Twenty-three recipients (24.2%) developed de novo DSA within 1-year posttransplant. Recipients who developed DSA had significantly lower quantities of peripheral CXCR5IFN-γCD8 T cells (P = 0.01) and significantly lower ratios of CXCR5IFN-γCD8 T cell to combined CD4 Th1/Th2 cell subsets (IFN-γCD4 and IL-4CD4 cells; P = 0.0001) compared to recipients who remained DSA-negative over the first-year posttransplant.

CONCLUSIONS

Our data raise the possibility that human CXCR5IFN-γCD8 T cells are a homolog to murine CXCR5IFN-γCD8 T cells (termed antibody-suppressor CD8 T cells) and that the quantity of CXCR5IFN-γCD8 T cells (or the ratio of CXCR5IFN-γCD8 T cells to Th1/Th2 CD4 T cells) may identify recipients at risk for development of DSA.

Characterization of the C1q-Binding Ability and the IgG1-4 Subclass Profile of Preformed Anti-HLA Antibodies by Solid-Phase Assays

Humoral alloimmunity, particularly that triggered by preformed antibodies against human leukocyte antigens (HLA), is associated with an increased prevalence of rejection and reduced transplant survival. The high sensitivity of solid phase assays, based on microbeads coated with single antigens (SAB), consolidated them as the gold-standard method to characterize anti-HLA antibodies, ensuring a successful allograft allocation. Mean fluorescence intensity (MFI) provided by SAB is regularly used to stratify the immunological risk, assuming it as a reliable estimation of the antibody-level, but it is often limited by artifacts. Beyond MFI, other properties, such as the complement-binding ability or the IgG1-4 subclass profile have been examined to more accurately define the clinical relevance of antibodies and clarify their functional properties. However, there are still unresolved issues. Neat serum-samples from 20 highly-sensitized patients were analyzed by SAB-panIgG, SAB-IgG1-4 subclass and SAB-C1q assays. All 1:16 diluted serum-samples were additionally analyzed by SAB-panIgG and SAB-IgG1-4 subclass assays. A total of 1,285 anti-HLA antibodies were identified as positive, 473 (36.8%) of which were C1q-binding. As expected, serum-dilution enhanced the correlation between the C1q-binding ability and the antibody-strength, measured as the MFI (r_neat = 0.248 vs. r_diluted = 0.817). SAB-subclass assay revealed at least one IgG1-4 subclass in 1,012 (78.8%) positive antibody-specificities. Among them, strong complement-binding subclasses, mainly IgG1, were particularly frequent (98.9%) and no differences were found between C1q- and non-C1q-binding antibodies regarding their presence (99.4 vs. 98.5%; p = 0.193). In contrast, weak or non-C1q-binding subclasses (IgG2/IgG4) were more commonly detected in C1q-binding antibodies (78.9 vs. 38.6%; p < 0.001). Interestingly, a strong association was found between the C1q-binding ability and the IgG1 strength (r_IgG1dil = 0.796). Though lower, the correlation between the IgG2 strength and the C1q-binding ability was also strong (r_IgG2dil = 0.758), being both subclasses closely related (r_IgG1−IgG2 = 0.817). We did not find any correlation with the C1q-binding ability considering the remaining subclasses. In conclusion, we demonstrate that a particular profile of IgG subclasses (IgG1/IgG3) itself does not determine at all the ability to bind complement of anti-HLA antibodies assessed by SAB-C1q assay. It is the IgG subclass strength, mainly of IgG1, which usually appears in combination with IgG2, that best correlates with it.