The present study clearly demonstrates that continuous exposure to organic material alters innate immune responses. Thus, daily exposure to high levels of pathogen-associated molecular patterns (PAMPs) was associated with lower number of macrophages and lower levels of soluble CD14 (sCD14) and TLR2 (sTLR2) in sputum from farmers compared with unexposed healthy subjects. The expression of CD14 on sputum neutrophils and adhesion molecules (CD62L and CD162) on peripheral blood neutrophils was lower in farmers than in controls and exposure to organic dust and LPS increased sST2 levels in serum in the control subjects but not in the farmers. The increased concentration of blood monocytes and decreased expression of CD62L (L-selectin) and CD162 (P-selectin) on blood neutrophils in farmers suggest a systemic engagement. This is supported by previous findings of reduced surface expression of CD62L and CD162 on neutrophils following activation with TNF and leukotriene B4
. Reduced surface expression of CD62L on blood neutrophils has been found in other conditions characterized by systemic inflammation, such as type- 2-diabetes with micro-angiophaty 
. It has been shown that pig farmers have elevated serum levels of soluble L-selectin, probably due to increased shedding of membrane bound protein 
. Our findings are intriguing considering that recruitment of neutrophils into the blood and airways following acute exposure in a pig barn and into the airways following LPS-challenge are attenuated in pig farmers compared with controls 
. The causes for the impaired neutrophil migration in farmers may be a consequence of impaired release of CXCL8 
and reduced expression of adhesion molecules on blood neutrophils. The expression of CD62L and CD162 was reduced and CD11b expression was enhanced on sputum neutrophils compared with peripheral blood neutrophils which are most likely due to activation of neutrophils during the migration process during transition from blood into the airways. This activation seems to be independent of previous or ongoing exposure as we found a similar response in the regularly exposed farmers and naïve controls.
It has been suggested that sTLR2 has an important role as a negative or first line regulator of membrane bound TLR2 by binding TLR2 ligands, thereby limiting the activation of membrane bound TLR2 in order to reduce harmful host effects 
. Thus, adding recombinant sTLR2 to monocytic culture strongly attenuated cell responses to TLR2 stimuli 
. The reduced levels of sTLR2 in sputum in farmers are likely related to inhalation of high levels of TLR2 ligands which directly bind to produced sTLR2. Monocytes release sTLR upon activation 
and it could be anticipated that monocytes still release sTLR2 after transformation into macrophages which in turn indicates a causal relationship between the decreased numbers of macrophages in sputum the lower levels of sTLR2 in the farmers. It has not been clearly shown that neutrophils, like monocytes 
, release sTLR2 upon activation. However, it could be speculated that also neutrophils release TLR2 which may explain the low expression on sputum neutrophils.
Chronic exposure in pig barn environment seems to affect the CD14 expression as the expression on sputum neutrophils was lower in farmers than in controls. Farmers have signs on neutrophilic airway inflammation 
and it could be speculated that the reduced CD14 expression on sputum neutrophils from farmers is caused by increased levels of elastase known to be increased in neutrophilic inflammation. Neutrophil elastase has a proteolytic effect on membrane bound CD14 by releasing the receptor into extracellular medium 
. As farmers have increased proportion of blood Th2 cells producing IL-4 and IL-13 
, this may contribute to the explanation as Th2 cytokines, e g IL-4, have the ability to reduce surface expression of CD14 
We observed decreased sputum levels of sCD14 in the farmers. Farmers regularly inhale high amounts of LPS that binds to sCD14 which therefore may not be detectable with the ELISA method used. It might also be related to the decreased proportion of sputum macrophages in farmers as we observed a correlation between sCD14 and number of sputum macrophages indicating that macrophages may be an important producer of sCD14.
Increased levels of sST2 are induced by pro-inflammatory stimuli such as LPS and TNF 
. Serum levels of sST2 are increased in several inflammatory disorders 
and intravenous injection of LPS increases serum levels of sST2 dramatically in healthy subjects 
. The attenuated response in serum sST2 release after exposure in a pig house and LPS challenge in the farmers might be due to tolerance. As systemic cytokine responses are attenuated in farmers 
this may, in part, explain the weaker sST2 increase in serum following exposure in pig barns in this group.
We have, in a previous study, observed increased concentration of blood neutrophils in farmers compared to unexposed controls, a finding that was not confirmed in the present study. This might be due to differences in exposure between the participating farmers. There is heterogeneity in previous exposure within the group of pig farmers with great variation in the cumulative exposure depending on the duration of employment and length of the working days. Thus, the possible difference in pre-trial cumulative exposure and the small study groups (15 pig farmers and 15 controls) may have influenced the possibilities to detect differences between the groups. However, despite this our study revealed some relevant and important new findings.
In conclusion, farmers showed altered expression of adhesion proteins on blood neutrophils, altered expression and soluble levels of PRR in the airways and attenuated release of sST2 after exposure in a pig barn and after a bronchial LPS challenge. The altered response in the farmers is probably caused by chronic exposure in pig barn environment and may be of pathogenic importance in development of chronic airway diseases, such as chronic bronchitis and increased occurrence of colonization of bacteria in the airways, conditions that are frequently observed in this group