Use of a monoclonal rat anti-mouse Ig light chain (RAMOL-1) antibody reduces background binding in immunohistochemical and fluorescent antibody analysis

Rat monoclonal antibodies (MAb) directed to mouse Ig heavy and light chain determinants were produced. A rat anti-mouse light chain MAb (RAMOL-1) which bound to all (24/24) mouse Ig of the kappa light chain type and with varying strength to 4/4 lambda light chain-bearing Ig was evaluated as a general secondary reagent, together with two MAb that bound to the heavy chain of mouse IgG. They were conjugated with biotin or FITC and used in immunohistochemical and immunofluorescence assays to detect mouse monoclonal antibodies binding to antigens expressed in rat and human tissues and cells. As compared to commercially available polyclonal reagents, RAMOL-1 gave higher staining contrast by showing lower background staining and equal or higher staining of the primary MAb tested. This was a result of two main effects. First, crossreactivity with endogenous Ig and tissue type-specific determinants was eliminated. With polyclonal anti-mouse Ig reagents, binding to endogenous Ig was noted in vascular spaces and on Ig-bearing cells, and to rat gastric mucosa and epithelial tumor tissue in frozen tissue sections, even when diluted in high concentrations of serum homologous to the tissue. Second, binding of the secondary reagent was reduced to cells and tissues prone to have high nonspecific binding capability, such as monocytes/macrophages and formalin-fixed, paraffin-embedded tissue. Owing to unlimited and reproducible access to this homogeneous reagent, RAMOL-1 is used as second antibody to standardize the procedure used for immunohistochemical grading of human malignant tumors by determination of blood group antigen expression detected with mouse MAb.