Melioidosis is a potentially fatal disease caused by the bacterium Burkholderia 
pseudomallei. Individuals with subclinical melioidosis have no apparent clinical 
signs or symptoms, and are identified only by positive serology. The present 
study is the first to investigate cell-mediated immune (CMI) responses following 
in vitro stimulation with B. pseudomallei antigens in peripheral blood 
mononuclear cells (PBMC), collected under field conditions in Papua New Guinea 
(PNG) from individuals with exposure to B. pseudomallei (n = 13). While five had 
a clinical history of melioidosis (C(+)), the remaining individuals (n = 8) were 
seropositive, yet healthy with no clinical history of melioidosis (S(+)/C(-)). 
Proliferation and IFN-gamma production were significantly greater in lymphocyte 
cultures from S(+)/C(-) individuals compared to C(+) individuals (P < 0.001 and 
P < 0.05, respectively). These findings demonstrate that compared to C(+) 
patients, individuals with subclinical melioidosis have a stronger CMI response 
to B. pseudomallei antigens in vitro. Such a response may be essential for 
protection against disease progression.