Human IgM Inhibits the Formation of Titan-Like Cells in Cryptococcus neoformans

Abstract

Human studies have shown associations between cryptococcal meningitis and reduced IgM memory B cell levels, and studies in IgM- and/or B cell-deficient mice have demonstrated increased Cryptococcus neoformans dissemination from lungs to brain. Since immunoglobulins are part of the immune milieu that C. neoformans confronts in a human host, and its ability to form titan cells is an important virulence mechanism, we determined the effect of human immunoglobulins on C. neoformans titan cell formation in vitro. (i) Fluorescence microscopy showed normal human IgG and IgM bind C. neoformans. (ii) C. neoformans grown in titan cell-inducing medium with IgM, not IgG, inhibited titan-like cell formation. (iii) Absorption of IgM with laminarin or curdlan (branched and linear 1-3-beta-d-glucans, respectively) decreased this effect. (iv) Transmission electron microscopy revealed that cells grown with IgM had small capsules and unique features not seen with cells grown with IgG. (v) Comparative transcriptional analysis of cell wall, capsule, and stress response genes showed that C. neoformans grown with IgM, not IgG or phosphate-buffered saline (PBS), had decreased expression of chitin synthetase, CHS1, CHS2, and CHS8, and genes encoding cell wall carbohydrate synthetases α-1-3-glucan (AGS1) and β-1,3-glucan (FKS1). IgM also decreased expression of RIM101 and HOG1, genes encoding central regulators of C. neoformans stress response pathways and cell morphogenesis. Our data show human IgM affects C. neoformans morphology in vitro and suggest that the hypothesis that human immunoglobulins may affect C. neoformans virulence in vivo warrants further investigation.

X-linked immunodeficient (XID) mice exhibit high susceptibility to Cryptococcus gattii infection

“…Moreover, our data also highlight an important role of the B-1 cells during chronic infection, as seen by the higher IgM antibody titer compared with the antigen-specific IgG antibody titer between the XID and BALB/c mouse. B-1 cells are involved in fighting C. neoformans infection, also due to their production of natural IgM antibodies against the yeast capsular content, while B-2 cells could act in the production of antigen-specific IgG antibodies 22 , 67 , so B-1 cells ends up being responsible for a large part of IgM in the early stages of C. gattii , and therefore, as we reveled, this amount was compromised in the XID mice.…”

Disseminated Cryptococcal Disease in A Patient With Chronic Chylothorax and a Pleurovenous Catheter, a Case Report With Autopsy Findings

“…The role of hypogammaglobulinemia in our patient also played an essential role in increasing the risk of infection. In vitro studies have demonstrated that human IgM and immunoglobulin G (IgG) change the morphology of C. neoformans and that IgM inhibits titian-like cell formation, which can reduce virulence and the risk of disseminated disease [ 35 ] . Previous reports have described an association between hypogammaglobulinemia and increased risk for cryptococcal disease, including in HIV-infected patients [ 6 , 36–38 ] .…”

Associations between Cryptococcus Genotypes, Phenotypes, and Clinical Parameters of Human Disease: A Review

“…X-linked hyper-IgM immunodeficiency syndrome is caused by mutations that occur in the CD40 ligand gene which results in decreased levels of IgG and IgA and elevated IgM has been shown to increase susceptibility to infections including invasive and cutaneous cryptococcosis particularly observed in pediatric male patients [ 148 , 149 , 150 , 151 , 152 , 153 , 154 , 155 ]. In addition, in vitro assays using C. neoformans found that IgM but not IgG inhibited Titan cell formation, reduced capsule thickness, and decreased the expression of chitin synthetase genes ( CHS1 , CHS2 , CHS8 , α-1–3-glucan synthetase ( AGS1 ), and β-1,3-glucan synthetase ( FKS1 ) [ 156 ]. Furthermore, IgM reduced the expression of the stress response pathways RIM101 and HOG1 [ 156 ].…”

“…In addition, in vitro assays using C. neoformans found that IgM but not IgG inhibited Titan cell formation, reduced capsule thickness, and decreased the expression of chitin synthetase genes ( CHS1 , CHS2 , CHS8 , α-1–3-glucan synthetase ( AGS1 ), and β-1,3-glucan synthetase ( FKS1 ) [ 156 ]. Furthermore, IgM reduced the expression of the stress response pathways RIM101 and HOG1 [ 156 ]. There have been several studies suggesting a link with polymorphism in the Fc-gamma receptors of immunoglobulin genes and cryptococcosis [ 157 , 158 , 159 ].…”