Specific immunoglobulin A antibodies to a peptide subunit sequence of bacterial cell wall peptidoglycan.

Immunoglobulin A (IgA) antibodies in human sera with binding specificity for the C-terminal R-D-Ala-D-Ala sequence of the precursor peptide from bacterial cell wall peptidoglycan were detected by an enzyme-linked immunosorbent assay (ELISA). Specificity of the test system was proved by comparing the high binding of specific IgA to albumin-(D-Ala3) as an antigen with the failure to bind to albumin-(L-Ala3), by binding inhibition studies with L-Ala3, D-Ala3, or peptides with structural analogy to peptidoglycan peptide subunit peptides as inhibitors, and by excluding binding of peroxidase-labeled anti-human IgA to immunoglobulin classes others than IgA. Interference of rheumatoid factors of IgA class was excluded by an ELISA for assaying IgA-rheumatoid factor and by the fact that an IgA fraction essentially free of IgG and IgM was isolated from a serum reacting strongly positive in the ELISA for measuring specific IgA to the peptide subunit of peptidoglycan. This isolated IgA again exhibited binding specificity in the ELISA, thus corroborating the existence of specific IgA in human serum to the C-terminal R-D-Ala-D-Ala sequence of peptidoglycan precursor peptide. The existence of IgA antibodies with specificity for bacterial peptidoglycan was further proved by preadsorption of serum to peptidoglycans and subsequent measurement of specific IgA in the ELISA. Screening of human sera for IgA antibodies with specificity for R-D-Ala-D-Ala peptides revealed that specific antibodies directed against this sequence of bacterial cell wall peptidoglycan may be detected in several human sera.