Glycosylation of immunoglobulins

Glycoproteins are post-translationally modified proteins containing branched sugar structures referred to as glycans. One major class of glycoproteins are the immunoglobulins. Immunoglobulins play a key role in the adaptive immune response and require glycosylation in the Golgi apparatus before release into circulation. The size, charge, and shape of glycans added to immunoglobulins determine half-life, effector function, and Fc receptor binding affinity. In the laboratory, classes of immunoglobulins can be isolated from patient blood and analyzed for glycan content using lectin, chromatographic, and mass spectrometry methods. In this thesis, glycans from human IgG and IgM are analyzed for changes during health and disease states. We elected to study Lyme disease because the host response to Lyme disease had not been characterized using a glycomic approach. We hypothesized that the host antibody glycomic response to acute Lyme disease would reveal more information about how the host immune response is mounted. Further, we hypothesized that this study could reveal mechanisms of host adaptive immune system evasion by the Lyme disease bacterium. We first asked: what immunoglobulin glycan alterations are associated with acute Lyme disease infection in humans? Here, we demonstrate IgG and IgM glycans produced in response to acute Lyme disease contain elevated levels of galactose and sialic acid. Next, we link the changes in immunoglobulin glycan profiles observed during acute Lyme disease to the functional impairment of IgG to signal for antibody-dependent cellular cytotoxicity. Following antibiotic therapy for Lyme disease, we demonstrate antibody glycosylation decreased the sialic acid content to reflect the resolution of Lyme disease. Later in this thesis, we characterize the glycan-associated serological response to COVID-19 by studying hospitalized patients infected from the Immunophenotyping assessment in a COVID-19 cohort. Studies of the COVID-19 glycomic response were previously limited to IgG glycans. We sought to characterize the glycomic response of both IgG and IgM to gain more insight into the adaptive immune response during COVID-19. We hypothesized that glycans on IgM would stratify patients with nonsevere versus severe COVID-19 better than IgG glycans. Further, we hypothesized that IgM glycans characterization would provide more insight into the acute host response to the viral infection. Here, we report significant differences in the IgG and IgM glycan profiles - particularly the ability of IgM glycans to aid in the stratification of nonsevere versus severe COVID-19 patients. We next hypothesized that the IgM glycans were modulating IgM- directed complement deposition. To test this, we developed an antigen-specific assay using purified IgM with and without sialidase digestions. Using this assay, we demonstrate a potential mechanism of the sialic acid glycans on IgM to promote complement deposition in severely infected COVID-19 patients. In conclusion, this thesis examines immunoglobulin glycosylation during human adaptive immune responses and links these glycan changes to modulated immunoglobulin effector function. In the case of acute Lyme disease, the sialic acid on antigen-specific IgG responding to Lyme disease impairs the induction of ADCC signaling. This impairment could be one mechanism of host immune evasion benefiting the Lyme disease bacterium. In contrast, elevated levels of sialic acid on IgM isolated from severe COVID-19 infections were associated with increased rates of complement deposition. Our data suggest this elevated IgM sialic acid could promote acute respiratory distress due to elevated complement deposition in the lung epithelia and endothelia of severe COVID-19 patients. Future studies should determine which pathways and cytokines are responsible for shifting the glycosylation of IgG and IgM during acute immune responses.