Activation of human dendritic cells is modulated by components of the outer membranes of Neisseria meningitidis

Abstract

Neisseria meningitidis serogroup B is a major cause of life-threatening meningitis and septicemia worldwide, and no effective vaccine is available. Initiation of innate and acquired immune responses to N. meningitidis is likely to be dependent on cellular responses of dendritic cells (DC) to antigens present in the outer membrane (OM) of the meningococcus. In this study, the responses of human monocyte-derived DC (mo-DC) to OM isolated from parent (lipopolysaccharide [LPS]-replete) meningococci and from a mutant deficient in LPS were investigated. Parent OM selectively up-regulated Toll-like receptor 4 (TLR4) mRNA expression and induced mo-DC maturation, as reflected by increased production of chemokines, proinflammatory cytokines, and CD83, CD80, CD86, CD40, and major histocompatibility complex (MHC) class II molecules. In contrast, LPS-deficient OM selectively up-regulated TLR2 mRNA expression and induced moderate increases in both cytokine production and expression of CD86 and MHC class II molecules. Preexposure to OM, with or without LPS, augmented the allostimulatory properties of mo-DC, which induced proliferation of naive CD4+ CD45RA+ T cells. In addition, LPS-replete OM induced a greater gamma interferon/interleukin-13 ratio in naive T cells, whereas LPS-deficient OM induced the reverse profile. These data demonstrate that components of the OM, other than LPS, are also likely to be involved in determining the levels of DC activation and the nature of the T-helper immune response.

FIG. 1.

Phenotypic analysis of mo-DC activated by N. meningitidis OM. Data are shown for cell surface expression of CD83, CD80, CD86, CD40, HLA-DR, and CD54 on control untreated mo-DC or cells that had been stimulated for 24 h with a 1-μg/ml concentration of OM from the parent H44/76 strain (OM) or from the LPS-deficient mutant (pLAK-OM). Expression of the indicated markers is shown by the solid histograms, whereas cells stained with relevant isotype MAb are indicated by the open histograms. The numbers on each histogram correspond to the median fluorescence intensity of MAb staining. The results shown are from one donor and are representative of similar data obtained from experiments carried out with mo-DC from eight different donors.

FIG. 2.

Effects of N. meningitidis OM on chemokine and cytokine production by mo-DC. Cells were stimulated for 24 h with various concentrations of OM from the parent H44/76 strain (OM) or from the LPS-deficient mutant (pLAK-OM) and compared with control, unstimulated cells. The data shown are the maximal levels of secretion induced with a 1-μg/ml concentration of preparation(s). Results are presented as means ± SEs of data from five donors (IL-1β, RANTES, and MIP-1α), six donors (IL-8 and MIP-1β), or eight donors (IL-6, IL-10, IL-12p40, IL-12p70, and TNF-α). *, P < 0.05 (t test) compared to control cells.

FIG. 3.

Effect of N. meningitidis OM on mo-DC-driven T-cell proliferation. Immature mo-DC were treated for 24 h with the optimal 1-μg/ml concentration of OM from the parent H44/76 strain (OM) or from the LPS-deficient mutant (pLAK-OM) and were also left untreated in culture as controls. Graded numbers of mo-DC were cocultured with 10⁵ cells of purified CD4⁺ allogeneic T cells (A) or CD4⁺ CD45RA⁺ naive allogeneic T cells (B) per well. Results are for triplicate cultures from one donor and are representative of similar data obtained from experiments done with mo-DC from five different donors. *, P < 0.05 compared to control cells.

FIG. 4.

Activation of mo-DC by N. meningitidis OM promotes cytokine production by primed naive T cells. Allogeneic CD4⁺ CD45⁺ naive T cells were cocultured with mo-DC preexposed for 24 h to the optimal 1-μg/ml concentration of OM from the parent H44/76 strain (OM) or from the LPS-deficient mutant (pLAK-OM). Results are presented as mean cytokine production ± SE from experiments done with mo-DC from three different donors.

FIG. 5.

Expression of TLR2 and TLR4 mRNAs by mo-DC treated with meningococcal OM. Cells were left untreated or exposed to the optimal 1-μg/ml concentration of OM from the parent H44/76 strain (OM) or from the LPS-deficient mutant (pLAK-OM), or to pure meningococcal LPS, for 24 h. Results show the averages ± SEs from experiments with mo-DC from five donors, except for TLR2 with pLAK-OM treatment, where four donors were used. *, P < 0.05 (t test). #, one additional donor (data not shown) did not show a change in TLR2 mRNA expression following treatment.