Human
cancers express organ-specific neoantigens (OSNs) which elicit specific cellular immune responses in the
cancer patient, as demonstrated by leukocyte adherence inhibition (LAI), an in vitro immune response assay. A purified
protein of MW 40,000 (p40) exhibiting OSN (colon specific) activity was cleaved into specific
peptide fragments and their partial amino acid sequences determined. This information was used in the polymerase chain reaction (PCR) to obtain a 992 bp
cDNA clone (PCR-992) from a human
colon adenocarcinoma cell line (LS-180). By comparison of the predicted amino acid sequence of PCR-992 with the known sequence of p40
peptides, PCR-992 was shown to correspond to almost the entire coding region of p40. Nucleotide sequence analysis suggested that the
protein was mycoplasmal in origin due to its high A+T content (76%) and the presence of five in frame TGA
termination codons; at least two of the latter are actually read as
tryptophan, a known feature of mycoplasma translation. We have confirmed this origin by direct isolation of a contaminating mycoplasma species from the LS-180 cell line and demonstration that it could be hybridized with the PCR-992 probe. Northern and PCR analysis of
RNA preparations from the contaminated LS-180 cell line showed that p40 was part of the high affinity transport system operon of Mycoplasma hyorhinis (Dudler et al, EMBO J., 7: 3963-3970, 1988). Total
protein lysates of Mycoplasma hyorhinis cultivated without animal cells could elicit positive LAI responses when incubated with
cancer patient leukocytes but not with normal patient leukocytes. The organ-specific nature of the response was, however, not observed indicating that host cell-mycoplasmal interactions may play a role in determining the organ-specific nature of p40 seen with the LAI. The significance of these findings will be discussed in the context of previous thinking regarding the origin of OSNs.