Evidence by radioimmunoassay that mitogen-activated human blood mononuclear cells secrete significant amounts of light chain Ig unassociated with heavy chain.

The culture supernatant (CS) Ig of PWM-activated human blood mononuclear cells was quantitatively determined using a panel of nanogram-sensitive radioimmunoassays (RIAs) that separately measured IgG, IgM, IgA, total kappa Ig, and total lambda Ig. After initial RIA quantitation, separate CS aliquots were exposed to either a polyisotypic anti-heavy (H) chain or a nonimmune IgG solid-phase immunoabsorbent, and then reassayed for Ig content. The reassay results revealed that the anti-H chain-absorbed CS aliquots retained significant amounts of kappa and lambda Ig, but yet had a virtual absence of isotypic IgG, IgM, and IgA. Comparisons of the absorbed CS aliquots suggested that as much as one-fourth to one-third of the total secreted L chain Ig in PWM-activated cultures lacked RIA-detectable associated H chain. This unexpected finding of significant amounts of unbound L chain in mitogen-stimulated cultures raises important theoretical issues relative to the functional role of secreted free L chain and the prospects that free L chain levels may represent useful quantitative markers of B-cell stimulation.