MOLECULAR CHARACTERIZATION OF A PROTOZOAN PARASITE TARGET ANTIGEN RECOGNIZED BY NONSPECIFIC CYTOTOXIC-CELLS

The target cell antigen(s) on tumor cells and on protozoan parasites r ecognized by NK and nonspecific cytotoxic cells (NCC) has not yet been specifically identified. NCC may be the teleost equivalent of NK cell s and IL-2-activated NK cells. A ligand recognized by NCC has been ide ntified. It is expressed on both protozoan parasites and mammalian tum or target cells, In the present study, a protozoan parasite antigen (N K target antigen/NKTag/p46) was purified from Tetrahymena pyriformis a nd the entire amino acid sequence was deduced from cDNA. Soluble and p urified NKTag inhibited NCC lysis of human and mouse transformed targe t cells. Homology comparisons using Swissprot database revealed that N KTag is a novel protein. Molecular weight computation of the deduced s equence demonstrated that NKTag is a 48.17-kDa protein containing 422 amino acids with relatively high percentages of tyrosine and serine re sidues. Expression of NKTag on various mammalian tumor target cells, n ormal tissue, and T. pyriformis was determined using anti-multiple ant igenic peptide (MAP) monoclonal antibody (mab) 22A12 [generated agains t an N-terminal 20-mer (aa 61-80) of p46]. This mab bound to tissue-cu ltured and tumor cells (YAC-1, IM-9, NC-37, MOLT-4, and U937) with low levels of binding to fish, mouse, and equine cells. Studies were also done to determine if purified and iodinated NKTag bound specifically to NCC. Binding was saturable and specific. These data provide evidenc e that NCC recognize a target cell ligand which is found on both proto zoan and tumor cells. This may provide an explanation as to how NCC (i ncluding activated NK cells) recognize a vast array of targets in the absence of haplotype recognition and in spite of a diverse species of origin. (C) 1997 Academic Press.