Induction of Immunomodulatory Factors by LPS and Nontoxic Derivatives

Abstract

Bacterial lipopolysaccharide (LPS) has been demonstrated to produce immunostimulatory effects on a wide variety of immunologic responses. Many cellular mechanisms have been proposed to explain these phenomenona. However, only recently has it become apparent that many of the immune response enhancing activities of LPS involve the induction of immunoregulatory factors. We have studied several of these factors in an attempt to establish a unified concept of their role in the mechanism of action of LPS on immune functions. The LPS-induced factors which are examined here include such lympho- kines as myeloid colony stimulating factor (CSF) and migration inhibition factor (MIF). Antitumor effects of LPS were demonstrated to be mediated by a tumor resistance enhancing factor and by tumor necrotizing factor (TNF). The enhancement of specific antibody responses by LPS was also mediated by a macrophage-derived helper factor. In addition to studying the induction of each of these factors by LPS, we attempted to determine which position of the LPS molecule was responsible for these activities. A nontoxic polysaccharide-rich LPS hydrolytic product (PS) was able to induce the production of all of these factors to a degree similar to that of whole LPS. Thus, the PS moiety contained an active site for the induction of immunomodulatory factors.

Bacterial lipopolysaccharides (LPS) are well known for their toxic and potentially lethal effects to both animals and individual cells. These toxic effects have been conclusively shown to be due to the Lipid A moiety of the LPS. However, most of the recent investigations into the mechanisms of LPS activity have focused upon ways in which LPS can nonspecifically enhance a broad variety of immune responses so as to produce a broad range of potentially beneficial effects (1–9). LPS has been demonstrated to regulate many of these immuno- stimulatory effects through the induction of lymphokines, monokines and other immunoregulatory factors. Effects of LPS which may be mediated through the production of these factors include the adjuvant effect upon in vitro antibody responses (1, 2), tumor resistance enhancement (3, 4, 5), tumor necrotizing activity (3, 4, 6, 7), in vitro myeloid colony stimulation (5, 8, 9), as well as a wide variety of other activities.

In spite of all these potentially beneficial effects of LPS, the toxic effects of LPS continue to make it unacceptible for general use as an immunostimulatory therapeutic agent for humans. Clearly, before considering widespread clinical trials, the LPS would have to be treated in a manner such as to remove its toxic activity while maintaining the immunostimulatory activity. We have attempted to produce a product with these qualifications through the partial acid hydrolysis of LPS and the removal of the toxic Lipid A component. The resultant polysaccharide-rich (PS) fraction is completely nontoxic (8). This PS fraction contains active sites for the induction of many of the stimulatory effects of LPS including in vivo (10) and in vitro (11) adjuvant activity, colony stimulating factor (CSF) production (5, 8), protection from lethal irradiation (10), and some antitumor effects (5).

This paper reviews many of our studies into the immunomodulatory effects of LPS, with emphasis on demonstrating the ways in which the nontoxic PS fraction can parallel the activity of the whole LPS in the stimulation of these immune responses. A central theme of these studies has been the demonstration of the ability of both LPS and PS to induce the production of immunomodulatory factors which mediate the cellular interactions which are essential for enhanced immunologic responsiveness. The PS-induced factors which will be discussed include myeloid colony stimulating factor (CSF), tumor resistance em- hancing factor, tumor hemorrhagic factor (TNF), leukocyte migration inhibition factor (MIF), and antibody response helper factor.