Immunoglobulin A nephropathy is characterized by anticommensal humoral immune responses

Abstract

IgA nephropathy (IgAN) is a leading cause of kidney failure, yet little is known about the immunopathogenesis of this disease. IgAN is characterized by deposition of IgA in the kidney glomeruli, but the source and stimulus for IgA production are not known. Clinical and experimental data suggest a role for aberrant immune responses to mucosal microbiota in IgAN, and in some countries with high disease prevalence, tonsillectomy is regarded as standard-of-care therapy. To evaluate the relationship between microbiota and mucosal immune responses, we characterized the tonsil microbiota in patients with IgAN versus nonrelated household-matched control group participants and identified increased carriage of the genus Neisseria and elevated Neisseria-targeted serum IgA in IgAN patients. We reverse-translated these findings in experimental IgAN driven by BAFF overexpression in BAFF-transgenic mice rendered susceptible to Neisseria infection by introduction of a humanized CEACAM-1 transgene (B × hC-Tg). Colonization of B × hC-Tg mice with Neisseria yielded augmented levels of systemic Neisseria-specific IgA. Using a custom ELISPOT assay, we discovered anti-Neisseria-specific IgA-secreting cells within the kidneys of these mice. These findings suggest a role for cytokine-driven aberrant mucosal immune responses to oropharyngeal pathobionts, such as Neisseria, in the immunopathogenesis of IgAN. Furthermore, in the presence of excess BAFF, pathobiont-specific IgA can be produced in situ within the kidney.

Keywords: Chronic kidney disease; Cytokines; Immunoglobulins; Immunology; Nephrology.

Figure 1. Tonsil microbiota.

(A) The most abundant genus (>2× the relative abundance of the next most abundant genus) in tonsil swabs in patients with IgAN was Neisseria. Each color represents an individual genus, and y axis indicates the proportion of samples with that genus as the most abundant organism (χ², P = 0.18). (B) The relative abundance of Neisseria genus was significantly higher in IgAN compared with the nonrelated household-matched healthy control individuals (2-tailed unadjusted t test P = 0.002).

Figure 2. The anti-Neisseria response in plasma of patients with IgA nephropathy.

Patients with IgAN exhibited exaggerated IgA-biased anti-Neisseria responses to both pathogenic and nonpathogenic Neisseria species. Neisseria-specific antibodies were evaluated across a panel of 4 N. meningitidis (Nme) strains (90/18311, H4476, 208, and 860800) and 4 commensal strains (N. lactamica, N. flavescens, N. sicca, and N. cinerea). Mean and SD shown, Mann-Whitney U test, 2-tailed P value).

Figure 3. The effect of BAFF overexpression on sterilizing immunity against N. meningitidis 90/18311.

(A) Rate of Nme colonization according to genotype during naive infection. (B) Bacterial burden 24 hours after secondary infection in littermate controls. Naive hC-Tg mice were used as a comparison for bacterial burden. (C) At 24 hours after nasal infection, B × hC-Tg mice did not exhibit an exaggerated anti-Neisseria IgA response in the nasopharynx. No anti-Neisseria IgG was detected in mice (not shown). (D) After nasal infection, hC-Tg mice exhibited enhanced systemic anti-Nme IgG response but B × hC-Tg mice did not (1-way ANOVA F 5.0, P < 0.01, adjusted P < 0.05 for indicated comparisons). (E) B × hC-Tg mice exhibited an enhanced systemic anti-Nme IgA response with 10-fold anti-Nme IgA production (1-way ANOVA F 2.6, P = 0.07). (F) Ratio of anti-Nme IgA/IgG revealed IgA-biased systemic response in the B × hC-Tg mice (1-way ANOVA F 2.3, P = 0.09, unadjusted P value as shown). All tests 1-way ANOVA with Tukey’s test; adjusted P value, mean, and SD shown.

Figure 4. Kidney pathology and IgA expression in experimental IgA nephropathy.

(A) Semiquantitative scoring revealed mesangial expansion in mice that overexpressed BAFF. (B) Representative images corresponding to mesangial matrix scoring (PAS stain, 40× original magnification) in BAFF-Tg × hCEACAM⁺ (a), BAFF-tg × hCEACAM ⁺ (B × hC-Tg) (b), BAFF-wt × hCEACAM^–/– (c), and BAFF-wt × hCEACAM1^–/– (d). Red arrow shows cellular proliferation; blue arrow shows mesangial expansion. (C) Representative sections showing IgA staining by immunohistochemistry (same order as B), confirming mesangial deposition of IgA. (D) Quantitative evaluation of IgA RNA expression (secreted splice form) in kidney tissue. Kidneys obtained from double transgenic mice (B × hC-Tg) demonstrated the highest degree of IgA mRNA expression (Dunn’s adjusted P < 0.05 for comparisons with other groups by Kruskal-Wallis 1-way ANOVA test). Total original magnification, ×40. Mean and SD shown.

Figure 5. Detection of anti–N. meningitidis IgA-secreting cells in the kidneys of BAFF-transgenic mice nasally infected with N. meningitidis via hCEACAM-1.

(A) Representative photos of an ELISPOT assay developed for detection of Nme-reactive IgA-producing cells. (B) Anti-Nme-specific IgA–antibody-secreting cells (igA-ACS) were detected predominantly in kidneys of BAFF-Tg × hCEACAM⁺ (B × hC-Tg) mice (mean and SD shown, 1-way ANOVA with Tukey’s test, adjusted P = 0.01 for all comparisons indicated).