Prothymosin alpha (proTalpha) is a 109 amino acid long polypeptide presenting 
distinct immunoenhancing activity in vitro and in vivo. Recent reports suggest 
that in apoptotic cells, proTalpha is cleaved by caspases at its 
carboxy(C)-terminus generating potentially bioactive fragments. In this study, 
we identified the peptide segment of proTalpha presenting maximum 
immunomodulatory activity. Calf thymus proTalpha was trypsinised, and the five 
fragments produced (spanning residues 1-14, 21-30, 31-87, 89-102 and 103-109) 
were tested for their ability to stimulate healthy donor- and cancer 
patient-derived peripheral blood mononuclear cell (PBMC) proliferation in 
autologous mixed lymphocyte reaction (AMLR), natural killer and 
lymphokine-activated killer cell activity, intracellular production of perforin, 
upregulation of adhesion molecules and CD25 expression. ProTalpha(89-102) and 
proTalpha(103-109) significantly fortified healthy donor-lymphocytes' immune 
responses to levels comparable to those induced by intact proTalpha. These 
effects were more pronounced in cancer patients, where peptides 
proTalpha(89-102) and proTalpha(103-109) partly, however significantly, restored 
the depressed AMLR and cytolytic ability of PBMC, by simulating the biological 
activity exerted by intact proTalpha. ProTalpha(1-14), proTalpha(21-30) and 
proTalpha(31-87) marginally upregulated lymphocyte activation. This is the first 
report showing that proTalpha's immunomodulating activity can be substituted by 
its C-terminal peptide(s). Whether generation and externalization of such 
immunoactive proTalpha fragments occurs in vivo, needs further investigation. 
However, if these peptides can trigger immune responses, they may eventually be 
used therapeutically to improve some PBMC functions of cancer patients.