IgG isotype distribution of local and systemic immune responses induced by influenza virus infection

The IgG isotype profile of the influenza virus‐specific immune response was studied by quantitation of serum antibody (Ab) levels in correlation with the enumeration of antibody‐secreting cells (ASC) detected in the lung, spleen, mediastinal lymph nodes (MLN), Peyer's patches and bone marrow (BM). Distincst isotypic patterns for serum Ab and Ab produced by cells present at or close to the site of infection were found after primary or repeated infections. An elevated number of IgM ASC was found after primary challenge in the spleen, lung and MLN. In contrast, the site of IgA and IgG production is restricted to the lung and lymph nodes draining the site of infection. In these organs IgA, IgG2a and IgG1 ASC are found as a result of primary virus infection while viral challenge induces mostly activation of IgA‐producing cells and secretion of IgA to the lung lavage. In contrast, the majority (80‐90%) of Ab detected in the serum belong to the IgG2a subclass and their serum level is maintained at a high level during the whole period of the response. The relative level of virus‐specific serum IgG2a in correlation with the production of IgG2a Ab found predominantly in MLN and lung is highly dependent on the viral dose used for priming or challenge. As IgG2a ASC can be detected at relatively low numbers in the spleen and BM these results suggest that the production of the dominant IgG2a isotype of serum Ab occurs close to the viral challenge site. These data, however, point to distinct isotypic regulation in systemic versus local virus‐specific Ab responses.