Potential Activity of Carrageenan to Enhance Antibacterial Host-Defense Systems in Mice

We investigated the influences of carrageenan, generally known to be a selective cytotoxic agent for macrophages, on the antibacterial host-defense systems such as the chemotactic responses to heat-killed bacteria or lipopolysaccharide in the pleural cavity and interleukin-1 production by pleural macrophages. Twenty-four hours after intraperitoneal injection of 200 mg/kg of carrageenan, there were no significant differences in the total and differential cell counts in lavaged fluid from the pleural cavity in carrageenan-pretreated and control mice. After intrapleural injection of heat-killedKlebsiella pneumoniae DT-S or lipopolysaccharide of this strain, significantly greater amounts of leukocytes, predominantly neutrophils, accumulated at 24 hrs in the pleural cavity of carrageenan-pretreated mice compared with control mice (P<0.05). Pleural macrophages obtained from carrageenan-pretreated mice produced more interleukin-1 in response to heat-killedK. pneumoniae DT-S or lipopolysaccharide than the control. Pretreatment with carrageenan significantly prolonged survival of mice against intrapleural challenge withK. pneumoniae DT-S. All 13 control mice died within 48 of inoculation with 9.2×10⁶ organisms per mouse, whereas 11 of 13 carrageenan-pretreated mice survived (P<0.001). These results indicate that carrageenan pretreatment enhances the chemotactic responses to bacterial lipopolysaccharide in the pleural cavity and interleukin-1 production by pleural macrophages. In some experimental conditions, this polysaccharide may act as a macrophage priming agent, instead of macrophage blocker.