Calcium activation of tissue transglutaminase in radioligand binding and enzyme-linked autoantibody immunoassays in childhood celiac disease.
BACKGROUND
Conflicting data have been published concerning the effect of calcium on binding of autoantibodies to tissue transglutaminase (tTG) in celiac disease (CD).


METHODS
IgA-tTG and IgG-tTG were measured with radioligand binding assays (RBA) using human recombinant (hr) (35)S-tTG produced in lysate of rabbit reticulocytes and with guinea pig (gp) tTG ELISA in 51 CD children (median: 5.7 years) and 35 controls (median: 2.2 years). Assays were performed with and without calcium.


RESULTS
In hr-tTG RBA, IgA-tTG levels remained unchanged after calcium detecting 50/51 CD children and 1/35 controls (p<0.0001). IgG-tTG levels decreased with calcium (p<0.0001) in CD children and detected 48/51 with and 49/51 without calcium as compared to 1/35 controls (p<0.0001). In gp-tTG ELISA, levels increased with calcium (p<0.0001) making it possible to detect an additional three to a total of 50/51 with IgA-tTG and 13 to 39/51 CD children with IgG-tTG compared to 4/35 and 8/35 controls (respectively, p<0.0001). Rabbit reticulocytes displayed calcium-dependent tTG activity.


CONCLUSIONS
Calcium increased binding of IgA-tTG and IgG-tTG in the ELISA test. The reverse effect observed in RBA may be explained by competitive binding between calcium activated native rabbit reticulocyte tTG and hr (35)S-tTG. tTG autoantibody assays may need taking calcium into account for accurate diagnostic sensitivity and specificity for CD.
Agardh
Daniel
D
Department of Pediatrics, University Hospital MAS, Lund University, S-205 02, Malmö, Sweden. daniel.agardh@home.se
Roth
Bodil
B
Lernmark
Ake
A
Stenberg
Pål
P
D000293:Adolescent; D000328:Adult; D001323:Autoantibodies / Q000097:blood; D002118:Calcium / Q000276:immunology*; D002446:Celiac Disease / Q000175:diagnosis* / Q000201:enzymology* / Q000276:immunology; D002648:Child; D002675:Child, Preschool; D003937:Diagnosis, Differential; D004797:Enzyme-Linked Immunosorbent Assay / Q000379:methods*; D005260:Female; D019204:GTP-Binding Proteins / Q000276:immunology*; D006801:Humans; D007070:Immunoglobulin A / Q000097:blood; D007074:Immunoglobulin G / Q000097:blood; D007223:Infant; D008297:Male; D000091345:Protein Glutamine gamma Glutamyltransferase 2; D011869:Radioligand Assay / Q000379:methods*; D012680:Sensitivity and Specificity; D011503:Transglutaminases / Q000276:immunology*
D001323:Autoantibodies; D007070:Immunoglobulin A; D007074:Immunoglobulin G; D000091345:Protein Glutamine gamma Glutamyltransferase 2; D011503:Transglutaminases; D019204:GTP-Binding Proteins; D002118:Calcium