Simultaneous immunofluorimetric quantitation of IgM and IgG in single splenic lymphoid cells after immunization

The amount of IgM and IgG was determined in individual mouse spleen lymphocytes by double immunofluorimetry after "staining" with dimethylamino-naphthaline sulfonyl chloride-labeled anti-IgM and fluorescein isothiocyanate-labeled anti-IgG. During a single immune response against sheep erythrocytes there was an inverse relationship between the amounts of these two immunoglobulins in individual cells. There was an early steep increase of the cellular amount of IgM during the first 3 days after immunization. Subsequently IgG increased exceeding the initial IgG levels while IgM decreased. Most cells containing high amounts of IgM had a low IgG content and vice versa. The present results directly demonstrate a quantitative switch from IgM to IgG in individual cells on the single cell level after an antigenic stimulus. This indicates simultaneous active production of both immunoglobulins in these cells. For an explanation of this phenomenon differential mRNA splicing rather than DNA rearrangement as an underlying process for immunoglobulin expression may be proposed in these early B-cells.