Plaque-associated endogenous IgG and its impact on immunohistochemical detection of mouse monoclonal IgG antibodies in mouse models of Alzheimer's disease

Experimental studies for Alzheimers disease (AD) have largely depended on transgenic mice with {beta}-amyloidosis. Here, we report plaque-associated endogenous immunoglobulin G (PA-IgG) and its impact on indirect immunohistochemical detection of mouse monoclonal IgG antibodies (Ms monoclonal IgG Abs) in the brain of AD mouse models. Immunostaining for Ms IgG in AD mouse models demonstrated endogenous IgG in the brain parenchyma accumulated on microglia associated with amyloid {beta} (A{beta}) plaques and/or A{beta} plaques themselves. This PA-IgG caused robust off-target binding of secondary Abs against Ms IgG (H+L) in indirect immunohistochemistry using Ms monoclonal IgG Abs. Blocking with Fab fragments of anti-Ms IgG (H+L) Ab was not effective against off-target binding. Unexpectedly, we found that secondary Abs that specifically recognize Ms IgG1, 2a, 2b, and 3 did not cause off-target binding on frozen brain sections of AppNL-G-F/NL-G-Fmice, and enabled specific labeling of Ms monoclonal IgG Abs in the AD mouse model brains. We further demonstrated that indirect detection with a conventional secondary Ab against Ms IgG (H+L) Ab could lead to erroneous conclusions regarding A{beta} plaque burden and phosphorylated tau accumulation in AppNL-G-F/NL-G-Fmice, and the use of Ms IgG subclass specific secondary Abs allowed to avoid the inevitable impediment caused by the endogenous IgG accumulation. Specific indirect detection of Ms monoclonal IgG Abs in AD mouse models by the use of secondary Abs against Ms IgG subclass would accelerate AD research by expanding the choice of Abs available for histochemical analysis in AD studies.