Lipopolysaccharide (LPS) is a common component of gram negative bacteria cell walls which acts as a potent inducer of cytokine release from human monocytes. The purpose of this project was to isolate and identify the protein(s) on human monocytes which bind LPS and initiate the release of cytokines from these cells. Using sepharose beads which have been covalently crosslinked with LPS, we isolated two proteins from human monocytes lysates, which appear to bind specifically to the LPS coated beads and not the control beads. The molecular weight of these proteins was approximately 73 and 85 kd. The 73 kd protein appeared to be present in greater abundance. Two other groups, using different techniques, have likewise isolated a protein of approximately 73 kd which appears to bind LPS. Western blotting studies indicate that this 73 kD protein is probably heat shock protein 70. Although hsp70 is found in great abundance inside cells there are some indications that this and other heat shock proteins can also be found on the surface of some cells. Furthermore we found that the addition of purified hsp 70 to monocytes in culture can either suppress or enhance LPS induced TNF and IL-1 release depending on the concentration of LPS used. When low concentrations of LPS (<1 ng/ml) are added, hsp70 augements the amount of TNF produced. However, when hsp70 is added with higher concentrations of LPS (>50ng/ml), it appears to suppress the TNF release. Others have reported that LPS stimulation occurs via a CD14-dependent pathway at low concentrations and a CD14-independent pathway at higher concentrations of LPS. We hypothesize that at low LPS concentrations (when the CD14-dependent pathway is used) LPS binds to CD14 on the cell surface which then transfers the LPS to a hsp70-like molecule which may act to internalize the molecule. The addition of exogenous hsp70 under these conditions could result in hsp70 binding the LPS and being """"""""carried to"""""""" the cell surface when LPS binds CD14 thus increasing the number of LPS receptors on the surface and increasing the responsiness of the cells to LPS. At higher LPS concentrations, LPS binding is CD14-independent and the exogenous hsp70 may act as a competative inhibitor. This work has been completed and submitted for publication.
