Conversion of TBAb response to TSAb response by anti-human IgG antibody

Previously, we reported the conversion phenomenon (CP) of thyroid blocking antibody (TBAb) to thyroid stimulating antibody (TSAb) by induced cAMP production during incubation of TBAb-bound porcine thyroid cells (PTC) with rabbit anti-IgG Ab. In the present experiment we examined the CP by TBAb-positive sera with high TSH binding inhibitor immunoglobulin (TBII) activity in primary hypothyroidism. Two patients with extremely high TBII patients; patient No.1 (35 yo male) with TSH 26.5μU/ml, TSAb negative, TBII 4,600 U/L, TBAb100% and patient No.2 (40 yo female) with TSH 4.5μU/ml, TSAb negative, TBII 1,620 U/L, TBAb 99.8% were examined. Cyclic AMP production was examined by 2nd incubation (3h) of anti-IgG Ab with TBAb-bound PTC that was made by 1st incubation (0.5h) of TBAbpositive serum and PTC. When sera (0.001-0.05 ml) of patient No.1 and No.2 were tested, cAMP production showed 980- 3,700% and 570-3,000% in a dose-dependent manner, respectively. Cyclic AMP production was also observed by anti- IgG fragments Ab [(Fab')2, Fab and light chain]. Cyclic AMP production by anti-F(ab')2 was higher than anti-Fab Ab, and cAMP by anti-κ Ab was significantly higher (>3 fold) than anti-λ Ab. Cyclic AMP production by TBAb-positive sera with high TBII activity (35-270 U/L) showed a correlation with serum TBII activity (R=0.76). The fact that all high TBAb-positive sera show the CP of TBAb to TSAb suggests that TSAb activity may be present in TBAb molecule and TBAb may be the precursor of TSAb.