Prostaglandins in experimental syphilis: treponemes stimulate adherent spleen cells to secrete prostaglandin E2, and indomethacin upregulates immune functions

Treponema pallidum protein Tp47 induced prostaglandin E2 to inhibit the phagocytosis in human macrophages

BACKGROUND

During Treponema pallidum (T. pallidum) infection, the host's immune system actively engages in pursuit and elimination of T. pallidum, while T. pallidum skillfully employs various mechanisms to evade immune recognition. Macrophages exhibit incomplete clearance of T. pallidum in vitro and the underlying mechanism of how T. pallidum resists the attack of macrophage remains unclear.

OBJECTIVES

To investigate the effect of T. pallidum membrane protein Tp47 on the phagocytosis of macrophages.

METHODS

THP-1-derived macrophages were used to investigate the role of Tp47 in the secretion of Prostaglandin E2 (PGE2) in macrophages and the mechanism by which Tp47 induced the production of PGE2, as well as the impact of PGE2 on the macrophage's phagocytosis.

RESULTS

Tp47 (1-10 μg/mL) significantly inhibited the phagocytosis of latex beads and T. pallidum in macrophages (p ≤ 0.05). PGE2 production by macrophages could be induced by Tp47, and the phagocytic function of macrophages could be restored using PGE2 antibody. Tp47 produced PGE2 by activating the PERK/NF-κB/COX-2 pathway in macrophages. Inhibitors targeting PERK, NF-κB and COX-2, respectively, reduced the level of PGE2 and restored the phagocytic function of macrophages.

CONCLUSION

Tp47-induced PGE2 production via the PERK/NF-κB/COX-2 pathway contributed to macrophage phagocytosis inhibition, which potentially contributes to immune evasion during the T. pallidum infection.

Treponema pallidum-induced prostaglandin E2 secretion in skin fibroblasts leads to neuronal hyperpolarization: A cause of painless ulcers

BACKGROUND

Primary syphilis is characterized by painless ulcerative lesions in the genitalia, the aetiology of painless remains elusive.

OBJECTIVES

To investigate the role of Treponema pallidum in painless ulcer of primary syphilis, and the mechanisms underlying painless ulcers caused by T. pallidum.

METHODS

An experimental rabbit model of primary syphilis was established to investigate its effects on peripheral nerve tissues. Human skin fibroblasts were used to examine the role of T. pallidum in modulating neurotransmitters associated with pain and to explore the signalling pathways related to neurotransmitter secretion by T. pallidum in vitro.

RESULTS

Treponema pallidum infection did not directly lead to neuronal damage or interfere with the neuronal resting potential. Instead, it facilitated the secretion of prostaglandin E2 (PGE2) through endoplasmic reticulum stress in both rabbit and human skin fibroblasts, and upregulation of PGE2 induced the hyperpolarization of neurones. Moreover, the IRE1α/COX-2 signalling pathway was identified as the underlying mechanism by which T. pallidum induced the production of PGE2 in human skin fibroblasts.

CONCLUSION

Treponema pallidum promotes PGE2 secretion in skin fibroblasts, leading to the excitation of neuronal hyperpolarization and potentially contributing to the pathogenesis of painless ulcers in syphilis.

The ability of peripheral blood mononuclear cells (PBMC) of syphilitic patients to produce IL-2

The cell-mediated immune response of importance in protection against Treponema pallidum, is distinctly suppressed in some stages of the disease. This may be a result of decreased ability of cells to produce IL-2, or IL-2 absorption by different factors. The experiments were designed to evaluate the ability of peripheral blood mononuclear cells (PBMC) of patients with different stages of syphilis to produce IL-2, and to investigate the causes which could possibly limit its activity. The ability of the PBMC of syphilitic patients to produce IL-2 develops at the beginning of the disease, reaching a maximum in primary seropositive syphilis. In the next stages of the disease this capability is distinctly lowered. The lowest was in malignant syphilis and tabes dorsalis, i.e. during severe disease. Absorption of adherent cells from PBMC increased the ability of lymphocytes to produce IL-2. The highest level of this interleukin was observed at the stages of the disease where suppression was the deepest. Sera of both control and syphilitic patients contained IL-2 inhibitor. Its level was the highest in early and late latent syphilis where no symptoms of disease were present. In all syphilitic sera a distinctly elevated level of soluble IL-2 receptors (sIL-2R) was also found. Its high level was noted in sera of patients in which PBMC had the weakest ability to produce IL-2. These findings suggest that sIL-2R may be bound to IL-2 and in this way would lead to weakening of T cell function and of resistance against Treponema pallidum infection.

Long-term study of cell-mediated responses to Borrelia burgdorferi in the laboratory mouse

Borrelia burgdorferi infection of disease-susceptible (C3H) and -resistant (BALB) mice resulted in impaired proliferation to both T- and B-cell mitogens up to 30 days after inoculation. Interleukin-2 and -4 production was also impaired, paralleling the T-cell response to concanavalin A. Impaired lymphocyte proliferation could not be attributed to diminished numbers of T or B cells and was found to depend on the lymphoid organ (spleen or lymph node) examined. Prostaglandin production accounted for part of this immune dysfunction. Attempts to assess antigen-specific proliferation to B. burgdorferi were inconsistent, and delayed-type hypersensitivity responses were not detected. Adoptive transfer of T-enriched cells from chronically infected donors failed to prevent infection and disease development in recipient C3H mice. The current study emphasizes caution in the study of B. burgdorferi antigen-specific assays and argues against the role of a vigorous T-cell response in Lyme borreliosis in infected laboratory mice.

Evidence that autologous idiotypic regulation of anti-arginine-glycine-aspartic acid autoantibodies may influence development and progression of syphilitic lesions in infected rabbits

The 83-kDa receptor protein of Treponema pallidum (TpN83) recognizes and binds fibronectin (Fn) at the amino acid sequence RGD site. By using experimentally infected animals, we have demonstrated that immunoglobulin G antibodies to this antigen and autoantibodies to the RGD site of Fn are putative components of immune complexes. This, and other findings, led us to initially hypothesize that anti-idiotypes (anti-Id) of an anti-TpN83 response are autoantibodies to RGD. Alternatively, we reasoned that if anti-Fn autoantibodies played a role in the pathogenesis of syphilis, then down-regulation of such a response by the Id network might directly affect progression of the disease. To test the hypothesis, rabbits were immunized with either affinity-purified TpN83 antigen or the synthetic Fn-7 peptide, KYGRGDS, and subsequently challenged with T. pallidum. Compared with results obtained with unimmunized, control rabbits, accelerated lesion development was noted in the rabbits immunized with TpN83. Pronounced, though unexpected, differences with respect to lesion development and progression were noted in the animals immunized with Fn-7 and then challenged intravenously; a minimal number of lesions appeared with a delayed onset. These lesions, like the localized chancres seen following intradermal challenge, were smaller and minimally ulcerated, and they healed rapidly. The Fn-7-immunized rabbits all differed from the controls in that anti-Id to anti-RGD F(ab')2 were demonstrable within 4 weeks following infection; decreases in anti-Fn autoantibody levels were associated with concomitant increases in anti-Id levels. Immunoglobulin Gs (anti-Id) from these animals following elution from anti-RGD F(ab')2 immunoaffinity columns also reacted with affinity-purified TpN83 antigen in immunoassays. These results suggest that down-regulation of autoreactive clones by manipulation of the idiotypic network in experimental syphilis warrants further investigation.

The Th1/Th2-like switch in syphilitic infection: is it detrimental?

Organisms that cause chronic diseases have evolved mechanisms to evade those immune defenses that resolve the acute stage of infection (10, 12-14, 21, 22, 32, 35, 37, 38, 40, 42, 45-49, 53). Much is to be learned by specifically identifying the mechanisms underlying these evasive strategies. Important new insights will emerge in terms of immunoregulatory pathways. This in turn will facilitate vaccine development. A good example is leishmania infection. The acute stage of this disease is resolved by DTH-macrophage activation. Leishmanial components preferentially activate Th2 lymphocytes. As a consequence, Th1 effects are minimized and infection is exacerbated leading to chronicity (10, 14, 32). To overcome this negative tendency, leishmanial vaccines are administered in combination with exogenous gamma interferon (42). This selects for Th1 predominance and generates protective immunity. Syphilis exhibits many parallels to the other nine chronic diseases mentioned above. Similarities include an acute localized stage that readily heals, early clearance via DTH-macrophage activation, transient concomitant immunity during acute infection, development of macrophage suppression through PGE2 down-regulation, beneficial effects of exogenous gamma interferon, and elements of autoimmunity. Some of the complexities of immunoregulation during treponemal infection have just begun to be unraveled. It will be important to develop further insight into the Th1/Th2 switch especially as it relates to chronicity. Macrophages seem to be intimately involved in the mechanics of this switch, and their specific role needs further clarification. Whatever is learned about syphilis, as well as other chronic infections will contribute to a better understanding of the generalized pathways of immunoregulation.

Splenic T-lymphocyte functions during early syphilitic infection are complex

Immune regulation during syphilitic infection is extremely complex. This paper presents findings on the early events of T-cell activation following testicular infection in rabbits. Treponema pallidum was preincubated for 24 h with nonadherent spleen cells. After being washed to remove the organisms, these spleen cells were either stimulated with concanavalin A (ConA) to induce interleukin-2 (IL-2), or added to adherent cells that were then stimulated with lipopolysaccharide to induce IL-1. Preincubation with the treponemes up-regulated nonadherent cell functions. These sensitized cells increased their IL-2 production and augmented macrophage IL-1 synthesis. In sharp contrast, if this preincubation step was omitted, down-regulation was apparent. When T. pallidum was directly incubated with nonadherent cells in the presence of ConA, reduced levels of IL-2 were detected. Nonadherent cells from infected rabbits secreted soluble suppressive factors after 48 h of in vitro incubation; these factors inhibited ConA-induced IL-2 generation as well as ConA-induced lymphocyte proliferation. At least some of this suppressive activity was attributed to transforming growth factor. In addition, when T lymphocytes were depleted, less suppression was detected. Treponemes also inhibited ConA-induced T-cell proliferation, and monophosphoryl lipid A reversed this inhibitory effect. Since monophosphoryl lipid A neutralizes T-suppressor activity, these findings further suggest a role for T-suppressor activity during syphilitic infection. Finally, T. pallidum directly stimulated IL-2 synthesis when coincubated with phorbol myristate acetate. This agent reverses the prostaglandin E2 blockage of T-helper cell protein kinase C, a necessary second messenger signal for IL-2 synthesis. In summary, T-cell functions are extremely complex and represent a composite of both stimulation and down-regulation, which occur concurrently but to different degrees.

Syphilis vaccine: up-regulation of immunogenicity by cyclophosphamide, Ribi adjuvant, and indomethacin confers significant protection against challenge infection in rabbits

Many unsuccessful attempts have been made to develop effective vaccines against experimental syphilitic infection. The focus of this report was to evaluate newer approaches to up-regulate immune responses following immunization with Treponema pallidum. Rabbits were injected once on day 0 with heat-inactivated treponemes suspended in the Ribi adjuvant system containing monophosphoryl lipid A (MPL) and trehalose dimycolate; animals were challenged dermally on day 29 with viable organisms. Various up-regulating agents were then tested using this general immunization protocol. When rabbits were pretreated on day -2 with cyclophosphamide (CYC), no protection was apparent. CYC pretreatment exhibited some protection when combined with a daily course of indomethacin on days 29 to 36. When rabbits were injected on day 0, then given a boost of MPL alone on day +2 plus indomethacin on days 29 to 36, minor protection was again apparent. Excellent protection was achieved when the vaccine protocol involved a combination of CYC pretreatment on day -2, an MPL boost on day +2, and indomethacin on days 29 to 36. Ninety-two percent of the subsequent lesions were atypical as indicated by their flat appearance, small size, lack of ulceration, and rapid healing. Importantly, this vaccine regimen also decreased dissemination of T. pallidum to distant tissues. These results suggest a new perspective in understanding immune responses in syphilis. We propose that vaccination, like infection, generates immune down-regulation that counter-balances immune stimulation. THus, effective vaccines will depend on removal and/or neutralization of treponemal components that down-regulate immune reactivity.