There is evidence for both physiological functions of the natural 
neurotransmitter, acetylcholine, and pharmacological actions of the plant 
alkaloid, nicotine, on the development and function of the immune system. The 
effects of continuous exposure to nicotine over a 12-day course of fetal thymus 
organ culture (FTOC) were studied, and thymocytes were analyzed by flow 
cytometry. In the presence of very low concentrations of nicotine many more 
immature T cells (defined by low or negative TCR expression) and fewer mature T 
cells (intermediate or high expression of TCR) were produced. In addition, the 
numbers of cells expressing CD69 and, to a lesser extent, CD95 (Fas) were 
increased. These effects took place when fetal thymus lobes from younger (13-14 
days gestation) pups were used for FTOC. If FTOC were set up using tissue from 
older (15-16 days gestation pups), nicotine had little effect, suggesting that 
it may act only on immature T cell precursors. Consistent with an increase in 
immature cells, the expression of recombinase-activating genes was found to be 
elevated. Nicotine effects were partially blocked by the simultaneous addition 
of the nicotinic antagonist d-tubocurarine. Furthermore, d-tubocurarine alone 
blocked the development of both immature and mature murine thymocytes, 
suggesting the presence of an endogenous ligand that may engage nicotinic 
acetylcholine receptors on developing thymocytes and influence the course of 
normal thymic ontogeny.