Bacterial lipopolysaccharides activate immune suppressor cells in newborn mice

Selective impairment of B cell function by Neisseria meningitidis

Spleen cells from CBA/J mice infected with Neisseria meningitidis displayed depressed in vitro plaque-forming cell (PFC) responses to T-dependent (sheep red blood cell; SRBC) and T-independent (TNP-LPS, TNP-Ficoll) antigens. The inhibition was observed over a wide range of antigen concentrations. The decreased responsiveness of splenocytes from infected mice was due to a selective impairment of B-cell function since helper-T-cell activity was intact in infected mice as shown by the ability of T-enriched lymphocytes to cooperate with normal B-enriched lymphocytes in the generation of an anti-SRBC response, accessory macrophage function was preserved since adherent spleen cells from bacteria-injected mice were shown to produce normal or increased levels of IL-1 and were able to cooperate with normal non-adherent spleen cells in the generation of PFC against SRBC. Addition of peritoneal cells from normal animals or extraneous IL-1 both failed to restore normal PFC responses in cultures of splenocytes from infected mice. Finally, B-enriched lymphocytes from infected mice produced poor anti-SRBC responses when cultured with either Con A supernatant or T-enriched lymphocytes from normal or infected mice. Cell-mixing experiments failed to detect the presence of suppressor cells in cultures of unfractionated spleen cells or B-enriched lymphocytes from infected mice. Therefore, the immunological unresponsiveness associated with a Neisseria meningitidis infection was attributed to a meningococcus-induced defect(s) in B-cell function. In vivo polyclonal B-cell activation leading to clonal exhaustion did not play a major role in the depression of humoral responses since meningococcal infection induced little or no polyclonal Ig secretion.

Mitogenic activities of synthetic lipid A analogs and suppression of mitogenicity of lipid A

The effect of synthetic lipid A analogs on murine spleen cells was studied. The preparations represented D-glucosamine and D-glucosaminyl-beta 1,6-D-glucosamine disaccharide derivatives substituted in different combinations by ester- and amide-bound fatty acids and by phosphate groups. Significant mitogenic activity was demonstrated with a number of synthetic disaccharide preparations; however, their potency was lower than that of lipid A. The synthetic preparations were not mitogenic for spleen cells from C3H/HeJ mice. Furthermore, the mitogenicity of the synthetic preparations was abolished after binding with polymyxin B. The results indicate that for expression of mitogenicity, a phosphate group at position 1 of the reducing glucosamine and amide-bound acyloxyacyl residues are important factors. Some of the synthetic preparations containing the diglucosamine backbone and expressing relatively low mitogenicity suppressed B-cell mitogenicity of lipid A. Although these preparations were lytic for erythrocytes, they did not affect the viability of the splenic lymphocytes. Suppression was seen when the synthetic preparations were added simultaneously with or after the lipid A mitogen, but optimal suppression was expressed when the preparations were added to the system 3 h before lipid A. Washing of the cells before the addition of lipid A did not affect the results. The suppression was not due to the induction of suppressor cells by the synthetic preparations. The disaccharide preparations did not inhibit T-cell mitogenicity of concanavalin A. In contrast to the disaccharide preparations, the monosaccharide preparations suppressed mitogenicity of both lipid A and concanavalin A, probably because of their direct toxicity for lymphocytes.