The suppressive effects of monospecific goat anti-mouse globulins on primary immunoglobulin class-specific plaque-forming cell responses in mouse spleen cell cultures were investigated. Anti-µ suppressed responses in all immunoglobulin classes, whereas anti-γ1 and anti-γ2 suppressed the γ1 and γ2 responses but not γM or γA responses, and anti-γA suppressed only γA responses. The mechanism of action of the anti-µ was studied in detail because of its suppression of responses in all immunoglobulin classes.

The anti-µ was specific for µ-chain determinants; its activity was dose dependent, but was not mediated by killing cells with surface µ-chain determinants. Free γM but not γG myeloma proteins in solution effectively competed with µ-bearing cells for the anti-µ. An excess of anti-µ was necessary in the cultures for 48 hr to insure complete suppression of 5-day responses. However, after removal of excess anti-µ at 48 hr, responses could be stimulated by newly added antigen in cultures where incubation was prolonged to 7 days. Anti-µ was most effective when added at the initiation of cultures and had no suppressive effect when added at 48 hr. Excess antigen did not effectively compete with anti-µ for antigen receptors. Precursors of antibody-forming cells were shown to be the cell population where the suppressive activity of anti-µ was mediated.

The experiments suggest that anti-µ combines with µ-chain determinants in antigen-specific receptors on the surfaces of antibody-forming cell precursors, prevents effective stimulation by antigen and subsequent antibody production. To explain suppression of responses in all Ig classes by anti-µ, several models were proposed. It is not possible to determine from the data whether stimulation of precursor cells with γG or γA receptors requires concommitant stimulation of separate cells with only γM receptors, or whether cells bearing γM receptors are precommitted to or differentiate into cells capable of synthesis of other Ig classes, or whether receptors of γM and another Ig class are present on some virgin precursors or the second Ig receptor appears after antigenic stimulation.

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