Pfc

Secretorv ratfe (IgMxIO 6g ml'1) (mol cell 1h1x10")

Figure 1 Measurements of B cell recruitment, clonal expression and activation of antibody secretion: cellular PFC and linnoval (ELISA) assays generate different conclusions. Limiting dilution cultures of human blood or tonsil lymphocytes were stimulated with TNP-ovalbumin in the absence (■) or presence (□) of high-dose (100 p,g ovalbumin per 24 h) primed, purified T cells. Cultures were assayed by hemolytic plaque assay for the frequency of TNP-specific PFC precursor cells and for the clone size present in positive cultures. A TNP-specific ELISA was used to measure specific antibody accumulated in supernatants over the 1-week culture period.

found that high-dose carrier (e.g. ovalbumin)-primed T cells appeared to actually reduce the optimal response of blood or tonsil cell cultures by 94-98% (Figure 1, columns 1 and 2, log scale). Closer analysis revealed that the few clones escaping this apparent suppression had much larger cone sizes (4-8-fold, columns 3 and 4). However, when immunoassays (e.g. ELISA) were used to measure specific antibodies secreted in these cultures, the suppressive effect of high-dose carrier-primed T cells was no longer apparent, since in addition to differential promotion of clonal expansion, these regulatory cells promoted higher secretory rates in these clones (-2-3 x 104 molecules cell"1 h_l). These latter two parameters are closely linked and vary concordantly during B cell maturation. Thus, depending on the assay system employed, such experiments may generate literally opposite conclusions.

It is therefore clear that the choice of assay system for immune responses can dramatically affect understanding and interpretations: in the given example, the apparent suppressor function following high-dose carrier priming identified in the PFC assay is not really reducing an immune response. Instead, these T cells recruit a clonally restricted portion of the hapten-specific B cell repertoire and promote its maturation more effectively. It is interesting to note that severe reduction of such apparent suppressor cell function induced with high doses of protein antigen is typical for a number of autoimmune conditions.

See also: Antibodies, secretion; B lymphocytes; Erythrocytes; Immune response in vitro-, ELISPOT assay.

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