Anti-S-layer monoclonal antibodies impact Clostridioides difficile physiology.

Clostridioides difficile (C. difficile), a gram-positive anaerobic and spore-forming bacterium, is the leading cause of nosocomial antibiotic-associated diarrhea in adults which is characterized by high levels of recurrence and mortality. Surface (S)-layer Protein A (SlpA), the most abundantly expressed protein on the bacterial surface, plays a crucial role in the early stages of infection although the nature of its involvement in C. difficile physiology is yet to be fully understood. Anti-S-layer antibodies have been identified in the sera of convalescent patients and have been correlated with improved outcomes of C. difficile infection (CDI). However, the precise mechanisms by which anti-S-layer antibodies confer protection to the host remain unknown. In this study, we report the first monoclonal antibodies (mAbs) targeting the S-layer of reference strain 630. Characterization of these mAbs unraveled important roles for the S-layer protein in growth, toxin secretion, and biofilm formation by C. difficile, with differential and even opposite effects of various anti-SlpA mAbs on these functions. Moreover, one anti-SlpA mAb impaired C. difficile growth and conferred sensitivity to lysozyme-induced lysis. The results of this study show that anti-S-layer antibody responses can be beneficial or harmful for the course of CDI and provide important insights for the development of adequate S-layer-targeting therapeutics.

A monoclonal antibody collection for C. difficile typing ?

Clostridioides difficile is the leading cause of antibiotic-associated diarrhea and pseudomembranous colitis in adults. Various C. difficile strains circulate currently, associated with different outcomes and antibiotic resistance profiles. However, most studies still focus on the reference strain 630 that does not circulate anymore, partly due to the lack of immunological tools to study current clinically important C. difficile PCR ribotypes. The goal of this study was to generate monoclonal antibodies recognizing various epidemic ribotypes of C. difficile. To do so, we immunized mice expressing human variable antibody genes with the Low Molecular Weight (LMW) subunit of the surface layer protein SlpA from various C. difficile strains. Monoclonal antibodies purified from hybridomas bound LMW with high-affinity and whole bacteria from current C. difficile ribotypes with different cross-specificities. This first collection of anti-C. difficile mAbs represent valuable tools for basic and clinical research.

Is vaccine response to SARS-CoV-2 preserved after switching to anti-CD20 therapies in patients with multiple sclerosis or related disorders?

BACKGROUND: Although vaccination against SARS-CoV-2 is recommended prior to introducing anti-CD20 therapies, limited data are available regarding the evolution of post-vaccinal immunity. METHODS: This retrospective study compared anti-Spike antibody titres at 6 and 12 months from SARS-CoV-2 vaccination between patients vaccinated before switching to anti-CD20 ('Switch') and two control groups: (1) patients vaccinated under disease-modifying therapies (DMTs) other than fingolimod and anti-CD20 ('Other DMTs'); (2) patients vaccinated on anti-CD20 ('Anti-CD20'). Anti-Spike-specific T-cell responses were compared between 'Switch' and 'Anti-CD20' groups. RESULTS: Fifty-three patients were included in the 'Switch' group, 54 in the 'Other DMTs' group and 141 in the 'Anti-CD20' group. At 6 months, in the subset of patients who received a booster dose, the 'Switch' group had lower anti-Spike titres compared with the 'Other DMTs' group (median 241.0 IQR (88.0; 504.0) BAU/mL vs 2034 (1155; 4634) BAU/mL, p<0.001), and less patients in the 'Switch' group reached the protective threshold of 264 BAU/mL. The 'Switch' group had higher anti-Spike titres than the 'Anti-CD20' group (7.5 (0.0; 62.1) BAU/mL, p=0.001). Anti-Spike titres were not different between the 'Switch' and 'Other DMTs' groups before booster administration. These results were similar at 12 months. Spike-specific T-cell positivity was similar between the 'Switch' and 'Anti-CD20' groups at 6 and 12 months (60.4% vs 61.0%, p=0.53, and 79.4% vs 87.5%, p=0.31, respectively). CONCLUSIONS: Despite a primary vaccination performed before the first anti-CD20 cycle, our results suggest weaker immune responses at 6 and 12 months and decreased booster efficacy after introducing anti-CD20. Patients vaccinated prior to anti-CD20 introduction might falsely be considered as fully protected by vaccination.

Intestinal Candida albicans overgrowth in IgA deficiency.

BACKGROUND: Secretory IgA interacts with commensal bacteria, but its impact on human mycobiota ecology has not been widely explored. In particular, whether human IgA-deficiency is associated with gut fungal dysbiosis remains unknown. OBJECTIVES: Our goal was to study the impact of IgA on gut mycobiota ecology. METHODS: The Fungi-Flow method was used to characterize fecal, systemic, and maternal IgA, IgM, and IgG responses against 14 representative fungal strains (yeast/spores or hyphae forms) in healthy donors (HDs) (n = 34, 31, and 20, respectively) and to also compare gut mycobiota opsonization by secretory antibodies in HDs (n = 28) and patients with selective IgA deficiency (SIgAd) (n = 12). Stool mycobiota composition was determined by internal transcribed spacer gene sequencing in HDs (n = 23) and patients with SIgAd (n = 17). Circulating CD4+ T-cell cytokine secretion profiles were determined by intracellular staining. The impact of secretory IgA, purified from breast milk (n = 9), on Candidaalbicans growth and intestinal Caco-2 cell invasion was tested in vitro. RESULTS: Homeostatic IgA binds commensal fungi with a body fluid-selective pattern of recognition. In patients with SIgAd, fungal gut ecology is preserved by compensatory IgM binding to commensal fungi. Gut Calbicans overgrowth nevertheless occurs in this condition but only in clinically symptomatic patients with decreased TH17/TH22 T-cell responses. Indeed, secretory IgA can reduce in vitro budding and invasion of intestinal cells by Calbicans and therefore exert control on this pathobiont. CONCLUSION: IgA has a selective impact on Calbicans ecology to preserve fungal-host mutualism.

Relevance of kappa free light chains index in patients with aquaporin-4 or myelin-oligodendrocyte-glycoprotein antibodies.

BACKGROUND: The kappa free light chains index (κ-index) is increasing in importance as a fast, easy, cost-effective, and quantitative biomarker in multiple sclerosis (MS), which can replace cerebrospinal fluid (CSF)-restricted oligoclonal bands (OCB) detection. In previous studies, controls often included mixed patients with several inflammatory central nervous system disorders. The aim of the present study was to assess the κ-index in patients with serum aquaporin-4 (AQP4)-IgG or myelin-oligodendrocyte-glycoprotein (MOG)-IgG. METHODS: We analyzed CSF/serum samples of patients with AQP4-IgG or MOG-Ig and evaluated distinct κ-index cut-offs. We described clinical and magnetic resonance imaging (MRI) features of patients with the highest κ-index values. RESULTS: In 11 patients with AQP4-IgG, median κ-index was 16.8 (range 0.2; 63) and 6/11 (54.5%) had κ-index >12. Among 42 patients with MOG-IgG, 2 had low positive MOG-IgG titers, were ultimately diagnosed with MS, and had a markedly increased κ-index (54.1 and 102.5 respectively). For the remaining 40 MOG-IgG-positive patients the median κ-index was 0.3 (range 0.1; 15.5). Some 6/40 (15%) and 1/40 (2.5%) patients had a κ-index >6 and >12, respectively. None fulfilled MRI dissemination in space and dissemination in time (DIS/DIT) criteria and the final diagnosis was MOG-IgG-associated disease (MOGAD) for these 40 patients. Four of the 40 (10%) MOG-IgG-positive patients had OCB. CONCLUSION: While a marked increase in κ-index could discriminate MS from MOGAD, a low κ-index threshold could lead to confusion between MS and MOGAD or AQP4 antibody-positive neuromyelitis optica spectrum disorder.

T-independent responses to polysaccharides in humans mobilize marginal zone B cells prediversified against gut bacterial antigens.

Marginal zone (MZ) B cells are one of the main actors of T-independent (TI) responses in mice. To identify the B cell subset(s) involved in such responses in humans, we vaccinated healthy individuals with Pneumovax, a model TI vaccine. By high-throughput repertoire sequencing of plasma cells (PCs) isolated 7 days after vaccination and of different B cell subpopulations before and after vaccination, we show that the PC response mobilizes large clones systematically, including an immunoglobulin M component, whose diversification and amplification predated the pneumococcal vaccination. These clones could be mainly traced back to MZ B cells, together with clonally related IgA+ and, to a lesser extent, IgG+CD27+ B cells. Recombinant monoclonal antibodies isolated from large PC clones recognized a wide array of bacterial species from the gut flora, indicating that TI responses in humans largely mobilize MZ and switched B cells that most likely prediversified during mucosal immune responses against bacterial antigens and acquired pneumococcal cross-reactivity through somatic hypermutation.

Kappa Free Light Chain Biomarkers Are Efficient for the Diagnosis of Multiple Sclerosis: A Large Multicenter Cohort Study.

BACKGROUND AND OBJECTIVES: Kappa free light chains (KFLC) seem to efficiently diagnose MS. However, extensive cohort studies are lacking to establish consensus cut-offs, notably to rule out non-MS autoimmune CNS disorders. Our objectives were to (1) determine diagnostic performances of CSF KFLC, KFLC index, and KFLC intrathecal fraction (IF) threshold values that allow us to separate MS from different CNS disorder control populations and compare them with oligoclonal bands' (OCB) performances and (2) to identify independent factors associated with KFLC quantification in MS. METHODS: We conducted a retrospective multicenter study involving 13 French MS centers. Patients were included if they had a noninfectious and nontumoral CNS disorder, eligible data concerning CSF and serum KFLC, albumin, and OCB. Patients were classified into 4 groups according to their diagnosis: MS, clinically isolated syndrome (CIS), other inflammatory CNS disorders (OIND), and noninflammatory CNS disorder controls (NINDC). RESULTS: One thousand six hundred twenty-one patients were analyzed (675 MS, 90 CIS, 297 OIND, and 559 NINDC). KFLC index and KFLC IF had similar performances in diagnosing MS from nonselected controls and OIND (p = 0.123 and p = 0.991 for area under the curve [AUC] comparisons) and performed better than CSF KFLC (p < 0.001 for all AUC comparisons). A KFLC index of 8.92 best separated MS/CIS from the entire nonselected control population, with better performances than OCB (p < 0.001 for AUC comparison). A KFLC index of 11.56 best separated MS from OIND, with similar performances than OCB (p = 0.065). In the multivariate analysis model, female gender (p = 0.003), young age (p = 0.013), and evidence of disease activity (p < 0.001) were independent factors associated with high KFLC index values in patients with MS, whereas MS phenotype, immune-modifying treatment use at sampling, and the FLC analyzer type did not influence KFLC index. DISCUSSION: KFLC biomarkers are efficient tools to separate patients with MS from controls, even when compared with other patients with CNS autoimmune disorder. Given these results, we suggest using KFLC index or KFLC IF as a criterion to diagnose MS. CLASSIFICATION OF EVIDENCE: This study provides Class III evidence that KFLC index or IF can be used to differentiate patients with MS from nonselected controls and from patients with other autoimmune CNS disorders.

Lower disease activity but higher risk of severe COVID-19 and herpes zoster in patients with systemic lupus erythematosus with pre-existing autoantibodies neutralising IFN-α.

OBJECTIVES: Type-I interferons (IFNs-I) have potent antiviral effects. IFNs-I are also overproduced in patients with systemic lupus erythematosus (SLE). Autoantibodies (AAbs) neutralising IFN-α, IFN-ß and/or IFN-ω subtypes are strong determinants of hypoxemic COVID-19 pneumonia, but their impact on inflammation remains unknown. METHODS: We retrospectively analysed a monocentric longitudinal cohort of 609 patients with SLE. Serum AAbs against IFN-α were quantified by ELISA and functionally assessed by abolishment of Madin-Darby bovine kidney cell protection by IFN-α2 against vesicular stomatitis virus challenge. Serum-neutralising activity against IFN-α2, IFN-ß and IFN-ω was also determined with a reporter luciferase activity assay. SARS-CoV-2 antibody responses were measured against wild-type spike antigen, while serum-neutralising activity was assessed against the SARS-CoV-2 historical strain and variants of concerns. RESULTS: Neutralising and non-neutralising anti-IFN-α antibodies are present at a frequency of 3.3% and 8.4%, respectively, in individuals with SLE. AAbs neutralising IFN-α, unlike non-neutralising AAbs, are associated with reduced IFN-α serum levels and a reduced likelihood to develop active disease. However, they predispose patients to an increased risk of herpes zoster and severe COVID-19 pneumonia. Severe COVID-19 pneumonia in patients with SLE is mostly associated with combined neutralisation of different IFNs-I. Finally, anti-IFN-α AAbs do not interfere with COVID-19 vaccine humoral immunogenicity. CONCLUSION: The production of non-neutralising and neutralising anti-IFN-I antibodies in SLE is likely to be a consequence of SLE-associated high IFN-I serum levels, with a beneficial effect on disease activity, yet a greater viral risk. This finding reinforces the recommendations for vaccination against SARS-CoV-2 in SLE.

High-affinity autoreactive plasma cells disseminate through multiple organs in patients with immune thrombocytopenic purpura.

The major therapeutic goal for immune thrombocytopenic purpura (ITP) is to restore normal platelet counts using drugs to promote platelet production or by interfering with mechanisms responsible for platelet destruction. Eighty percent of patients with ITP possess anti-integrin αIIbß3 IgG autoantibodies that cause platelet opsonization and phagocytosis. The spleen is considered the primary site of autoantibody production by autoreactive B cells and platelet destruction. The immediate failure in approximately 50% of patients to recover a normal platelet count after anti-CD20 rituximab-mediated B cell depletion and splenectomy suggests that autoreactive, rituximab-resistant, IgG-secreting B cells (IgG-SCs) reside in other anatomical compartments. We analyzed more than 3,300 single IgG-SCs from spleen, bone marrow, and/or blood of 27 patients with ITP, revealing high interindividual variability in affinity for αIIbß3, with variations over 3 logs. IgG-SC dissemination and range of affinities were, however, similar for each patient. Longitudinal analysis of autoreactive IgG-SCs upon treatment with the anti-CD38 mAb daratumumab demonstrated variable outcomes, from complete remission to failure with persistence of high-affinity anti-αIIbß3 IgG-SCs in the bone marrow. This study demonstrates the existence and dissemination of high-affinity autoreactive plasma cells in multiple anatomical compartments of patients with ITP that may cause the failure of current therapies.

Neuroinflammatory Disease following Severe Acute Respiratory Syndrome Coronavirus 2 Infection in Children.

OBJECTIVE: To describe neurologic, radiologic and laboratory features in children with central nervous system (CNS) inflammatory disease complicating severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. STUDY DESIGN: We focused on CNS inflammatory diseases in children referred from 12 hospitals in the Paris area to Necker-Sick Children Reference Centre. RESULTS: We identified 19 children who had a history of SARS-CoV-2 infection and manifest a variety of CNS inflammatory diseases: encephalopathy, cerebellar ataxia, acute disseminated encephalomyelitis, neuromyelitis optica spectrum disorder, or optic neuritis. All patients had a history of SARS-CoV-2 exposure, and all tested positive for circulating antibodies against SARS-CoV-2. At the onset of the neurologic disease, SARS-CoV-2 PCR results (nasopharyngeal swabs) were positive in 8 children. Cerebrospinal fluid was abnormal in 58% (11/19) and magnetic resonance imaging was abnormal in 74% (14/19). We identified an autoantibody co-trigger in 4 children (myelin-oligodendrocyte and aquaporin 4 antibodies), representing 21% of the cases. No autoantibody was found in the 6 children whose CNS inflammation was accompanied by a multisystem inflammatory syndrome in children. Overall, 89% of patients (17/19) received anti-inflammatory treatment, primarily high-pulse methylprednisolone. All patients had a complete long-term recovery and, to date, no patient with autoantibodies presented with a relapse. CONCLUSIONS: SARS2-CoV-2 represents a new trigger of postinfectious CNS inflammatory diseases in children.

Pre-COVID-19 Immunity to Common Cold Human Coronaviruses Induces a Recall-Type IgG Response to SARS-CoV-2 Antigens Without Cross-Neutralisation.

The capacity of pre-existing immunity to human common coronaviruses (HCoV) to cross-protect against de novo COVID-19is yet unknown. In this work, we studied the sera of 175 COVID-19 patients, 76 healthy donors and 3 intravenous immunoglobulins (IVIG) batches. We found that most COVID-19 patients developed anti-SARS-CoV-2 IgG antibodies before IgM. Moreover, the capacity of their IgGs to react to beta-HCoV, was present in the early sera of most patients before the appearance of anti-SARS-CoV-2 IgG. This implied that a recall-type antibody response was generated. In comparison, the patients that mounted an anti-SARS-COV2 IgM response, prior to IgG responses had lower titres of anti-beta-HCoV IgG antibodies. This indicated that pre-existing immunity to beta-HCoV was conducive to the generation of memory type responses to SARS-COV-2. Finally, we also found that pre-COVID-19-era sera and IVIG cross-reacted with SARS-CoV-2 antigens without neutralising SARS-CoV-2 infectivity in vitro. Put together, these results indicate that whilst pre-existing immunity to HCoV is responsible for recall-type IgG responses to SARS-CoV-2, it does not lead to cross-protection against COVID-19.

BNT162b2 vaccine-induced humoral and cellular responses against SARS-CoV-2 variants in systemic lupus erythematosus.

OBJECTIVES: Our aim was to evaluate systemic lupus erythematosus (SLE) disease activity and SARS-CoV-2-specific immune responses after BNT162b2 vaccination. METHODS: In this prospective study, disease activity and clinical assessments were recorded from the first dose of vaccine until day 15 after the second dose in 126 patients with SLE. SARS-CoV-2 antibody responses were measured against wild-type spike antigen, while serum-neutralising activity was assessed against the SARS-CoV-2 historical strain and variants of concerns (VOCs). Vaccine-specific T cell responses were quantified by interferon-γ release assay after the second dose. RESULTS: BNT162b2 was well tolerated and no statistically significant variations of BILAG (British Isles Lupus Assessment Group) and SLEDAI (SLE Disease Activity Index) scores were observed throughout the study in patients with SLE with active and inactive disease at baseline. Mycophenolate mofetil (MMF) and methotrexate (MTX) treatments were associated with drastically reduced BNT162b2 antibody response (ß=-78, p=0.007; ß=-122, p<0.001, respectively). Anti-spike antibody response was positively associated with baseline total immunoglobulin G serum levels, naïve B cell frequencies (ß=2, p=0.018; ß=2.5, p=0.003) and SARS-CoV-2-specific T cell response (r=0.462, p=0.003). In responders, serum neutralisation activity decreased against VOCs bearing the E484K mutation but remained detectable in a majority of patients. CONCLUSION: MMF, MTX and poor baseline humoral immune status, particularly low naïve B cell frequencies, are independently associated with impaired BNT162b2 mRNA antibody response, delineating patients with SLE who might need adapted vaccine regimens and follow-up.

Perturbed Microbiota/Immune Homeostasis in Multiple Sclerosis.

OBJECTIVE: Based on animal models and human studies, there is now strong suspicion that host/microbiota mutualism in the context of gut microbial dysbiosis could influence immunity and multiple sclerosis (MS) evolution. Our goal was to seek evidence of deregulated microbiota-induced systemic immune responses in patients with MS. METHODS: We investigated gut and systemic commensal-specific antibody responses in healthy controls (n = 32), patients with relapsing-remitting MS (n = 30), and individuals with clinically isolated syndromes (CISs) (n = 15). Gut microbiota composition and diversity were compared between controls and patients by analysis of 16S ribosomal ribonucleic acid (rRNA) sequencing. Autologous microbiota and cultivable bacterial strains were used in bacterial flow cytometry assays to quantify autologous serum IgG and secretory IgA responses to microbiota. IgG-bound bacteria were sorted by flow cytometry and identified using 16S rRNA sequencing. RESULTS: We show that commensal-specific gut IgA responses are drastically reduced in patients with severe MS, disease severity being correlated with the IgA-coated fecal microbiota fraction (r = -0.647, p < 0.0001). At the same time, IgA-unbound bacteria elicit qualitatively broad and quantitatively increased serum IgG responses in patients with MS and CIS compared with controls (4.1% and 2.5% vs 1.9%, respectively, p < 0.001). CONCLUSIONS: Gut and systemic microbiota/immune homeostasis are perturbed in MS. Our results argue that defective IgA responses in MS are linked to a breakdown of systemic tolerance to gut microbiota leading to an enhanced triggering of systemic IgG immunity against gut commensals occurring early in MS.

Rapid decline of neutralizing antibodies against SARS-CoV-2 among infected healthcare workers.

There are only few data concerning persistence of neutralizing antibodies (NAbs) among SARS-CoV-2-infected healthcare workers (HCW). These individuals are particularly exposed to SARS-CoV-2 infection and at potential risk of reinfection. We followed 26 HCW with mild COVID-19 three weeks (D21), two months (M2) and three months (M3) after the onset of symptoms. All the HCW had anti-receptor binding domain (RBD) IgA at D21, decreasing to 38.5% at M3 (p < 0.0001). Concomitantly a significant decrease in NAb titers was observed between D21 and M2 (p = 0.03) and between D21 and M3 (p < 0.0001). Here, we report that SARS-CoV-2 can elicit a NAb response correlated with anti-RBD antibody levels. However, this neutralizing activity declines, and may even be lost, in association with a decrease in systemic IgA antibody levels, from two months after disease onset. This short-lasting humoral protection supports strong recommendations to maintain infection prevention and control measures in HCW, and suggests that periodic boosts of SARS-CoV-2 vaccination may be required.

When Therapeutic IgA Antibodies Might Come of Age.

BACKGROUND: Extensive efforts have been made in optimizing monoclonal immunoglobulin (Ig)G antibodies for use in clinical practice. Accumulating evidence suggests that IgA or anti-FcαRI could also represent an exciting avenue toward novel therapeutic strategies. SUMMARY: Here, we underline that IgA is more effective in recruiting neutrophils for tumor cell killing and is potently active against several pathogens, including rotavirus, poliovirus, influenza virus, and SARS-CoV-2. IgA could also be used to modulate excessive immune responses in inflammatory diseases. Furthermore, secretory IgA is emerging as a major regulator of gut microbiota, which impacts intestinal homeostasis and global health as well. As such, IgA could be used to promote a healthy microbiota in a therapeutic setting. Key messages: IgA combines multifaceted functions that can be desirable for immunotherapy.

IgA dominates the early neutralizing antibody response to SARS-CoV-2.

Humoral immune responses are typically characterized by primary IgM antibody responses followed by secondary antibody responses associated with immune memory and composed of IgG, IgA, and IgE. Here, we measured acute humoral responses to SARS-CoV-2, including the frequency of antibody-secreting cells and the presence of SARS-CoV-2-specific neutralizing antibodies in the serum, saliva, and bronchoalveolar fluid of 159 patients with COVID-19. Early SARS-CoV-2-specific humoral responses were dominated by IgA antibodies. Peripheral expansion of IgA plasmablasts with mucosal homing potential was detected shortly after the onset of symptoms and peaked during the third week of the disease. The virus-specific antibody responses included IgG, IgM, and IgA, but IgA contributed to virus neutralization to a greater extent compared with IgG. Specific IgA serum concentrations decreased notably 1 month after the onset of symptoms, but neutralizing IgA remained detectable in saliva for a longer time (days 49 to 73 post-symptoms). These results represent a critical observation given the emerging information as to the types of antibodies associated with optimal protection against reinfection and whether vaccine regimens should consider targeting a potent but potentially short-lived IgA response.

Systemic anti-commensal response to fungi analyzed by flow cytometry is related to gut mycobiome ecology.

BACKGROUND: Interest for the study of gut mycobiota in relation with human health and immune homeostasis has increased in the last years. From this perspective, new tools to study the immune/fungal interface are warranted. Systemic humoral immune responses could reflect the dynamic relationships between gut mycobiota and immunity. Using a novel flow cytometry technology (Fungi-Flow) to determine immunoglobulin (Ig) responses to fungi, we studied the relationships between gut mycobiota and systemic humoral anti-commensal immunity. RESULTS: The Fungi-Flow method allows a sensitive and specific measurement of systemic IgG responses against 17 commensal and environmental fungi from the two main divisions; Ascomycota and Basidiomycota. IgG responses exhibited a high inter-individual variability. Anti-commensal IgG responses were contrasted with the relative abundance, alpha-diversity, and intra-genus richness of fungal species in gut mycobiota of twenty healthy donors. Categorization of gut mycobiota composition revealed two differentiated fungal ecosystems. Significant difference of anti-Saccharomyces systemic IgG responses were observed in healthy donors stratified according to the fungal ecosystem colonizing their gut. A positive and significant correlation was observed between the variety of IgG responses against fungal commensals and intestinal alpha-diversity. At the level of intra-genus species richness, intense IgG responses were associated with a low intra-genus richness for known pathobionts, but not commensals. CONCLUSIONS: Fungi-Flow allows an easy and reliable measure of personalized humoral responses against commensal fungi. Combining sequencing technology with our novel Fungi-Flow immunological method, we propose that there are at least two defined ecosystems in the human gut mycobiome associated with systemic humoral responses. Fungi-Flow opens new opportunities to improve our knowledge about the impact of mycobiota in humoral anti-commensal immunity and homeostasis. Video Abstract.