Acquired resistance and expression of a protective humoral immune response in guinea pigs infected with Treponema pallidum Nichols

Host defense mechanisms in secondary syphilitic lesions: a role for IFN-gamma-/IL-17-producing CD8+ T cells?

Cell-mediated immunity is thought to be of critical importance in antisyphilitic host defense, but the exact mechanisms are still unknown. This fact is particularly true for HIV-infected persons with a deficit in CD4+ T-cell number. We therefore obtained lesional skin samples from HIV+ and HIV- patients with secondary syphilis at different time points of lesional age to search both for causative microorganisms and to characterize the inflammatory infiltrate. By doing so, we detected Treponema pallidum spirochetes with a much greater abundance in late lesions of HIV+ individuals compared with the HIV- cohort. The dominating inflammatory cells were T cells, macrophages, and neutrophils at all stages and plasma cells in older lesions. In HIV- persons, T cells consisted of equal numbers of CD4+ and CD8+ T-cells, whereas in HIV+ patients, the majority of T cells belonged to the CD8 lineage and produced both IFN-γ and IL-17. Regulatory T cells and Langerhans cells were reduced in these patients compared with their HIV- counterparts. Because of our observations, we propose that T cells of both the CD4 and CD8 lineage are needed for an at least partial protective antisyphilitic immunity. Compensation mechanisms in HIV+ individuals, such as an increase of Tc1/17 cells as well as a reduction in immunoregulatory Langerhans cells and T cells, apparently do not overcome the deficiencies in these patients to eliminate the spirochete.

The guinea pig as a model of infectious diseases

The words 'guinea pig' are synonymous with scientific experimentation, but much less is known about this species than many other laboratory animals. This animal model has been used for approximately 200 y and was the first to be used in the study of infectious diseases such as tuberculosis and diphtheria. Today the guinea pig is used as a model for a number of infectious bacterial diseases, including pulmonary, sexually transmitted, ocular and aural, gastrointestinal, and other infections that threaten the lives of humans. Most studies on the immune response to these diseases, with potential therapies and vaccines, have been conducted in animal models (for example, mouse) that may have less similarity to humans because of the large number of immunologic reagents available for these other species. This review presents some of the diseases for which the guinea pig is regarded as the premier model to study infections because of its similarity to humans with regard to symptoms and immune response. Furthermore, for diseases in which guinea pigs share parallel pathogenesis of disease with humans, they are potentially the best animal model for designing treatments and vaccines. Future studies of immune regulation of these diseases, novel therapies, and preventative measures require the development of new immunologic reagents designed specifically for the guinea pig.

Effect of passive immunization with purified specific or cross-reacting immunoglobulin G antibodies against Treponema pallidum on the course of infection in guinea pigs

Whole immune serum or highly purified immunoglobulin G (IgG) antibodies to Treponema pallidum exhaustively adsorbed with three strains of nonpathogenic treponemes (TPI-IgG) were used for passive immunization of inbred strain 2 guinea pigs before and after intradermal challenge with 3.4 x 10(7) virulent T. pallidum Nichols organisms. Before challenge, control animals received a similarly purified IgG fraction containing either a cocktail of antibodies against three nonpathogenic treponemes (NPTI-IgG) or IgG prepared from normal guinea pig serum (NGPS-IgG). The purified fractions contained both IgG1 and IgG2 isotypes. The antibody levels (detected by fluorescent treponemal antibody test and enzyme-linked immunosorbent assay) and molecular specificities (immunoblot) of sera obtained from recipient animals before infection reflected those of the purified fractions used for immunization. Three protocols of passive immunization were used. Whole immune serum containing specific and cross-reacting antibodies afforded better protection than TPI-IgG even though asymptomatic animals were not fully protected. A single intradermal injection (0.1 ml) of TPI-IgG or NPTI-IgG into one hind leg 22 h before infection at the same site provided relatively higher protection than multiple intravenous injections (total, 15 ml) of the respective individual preparations. Since purified NGPS-IgG injected in the same animals, into the opposite hind leg, failed to protect against the challenging infection, it is reasonable to assume that specific and cross-reacting antitreponemal antibodies of the IgG1 subclass, which in guinea pigs are homocytotropic, play a relevant role in local protection.

Experimental syphilis in guinea pig

The infrequent use of guinea pig in experimental syphilis, the not well genetically and immunologically characterized strains of animals originating from places with unspecified conditions of husbandry, and the various strains of Treponema pallidum used for infection provided inconsistent and discouraging results. For eight decades the rabbit has been the major animal model in studies of syphilis. However, the lack of readily available inbred strains of rabbits--necessary for adoptive transfer experiments--has been a stumbling block in revealing the mechanisms responsible for immunity, susceptibility, and resistance to T. pallidum infection. These difficulties have recently been overcome by demonstration of inbred strains susceptible to T. pallidum infection, paving the way to studies of adoptive immunity. The guinea pig may also be a better model than the rabbit for immunomanipulations (irradiation, injection with antibodies specific to various cell populations), allowing a closer insight into the immunopathologic mechanism operating during the course of syphilitic infection. The "rediscovery" of the guinea pig as a model for experimental syphilis and recent years of intensive studies justify a review summarizing older data and providing the most recent information. The authors, having first-hand experience with this model, will provide detailed information on (1) historical background; (2) course of infection with T. pallidum in inbred and outbred strains of guinea pigs; (3) the ID50 for various strains; (4) various routes of infection; (5) age and sex-dependent susceptibility to infection; (6) kinetic of the humoral response to specific and non-specific treponemal antigens; (7) appearance of autoantibodies and immune complexes; (8) cellular response, including lymphoproliferative response, macrophage inhibitory factor(s) production, chemotaxis and adoptive transfer of immunity by purified T cells; and (9) a complete list of references.

Enhanced primary resistance to Treponema pallidum infection and increased susceptibility to toxoplasmosis in T-cell-depleted guinea pigs

Strain 2 guinea pigs made T-cell deficient by thymectomy and irradiation and protected with syngeneic bone-marrow cells (TXB guinea pigs) have a surprisingly high level of resistance to cutaneous syphilis and to the dissemination of treponemes to the draining lymph node. Compared with normal euthymic controls infected with Treponema pallidum Nichols, syphilitic TXB guinea pigs developed fewer and less severe skin lesions and their lymph nodes contained lower numbers of treponemes. Associated with this evidence for enhanced innate resistance was the ability of the TXB host to produce, during each test interval of a primary infection, more antitreponemal antibodies than that of their euthymic counterparts. Similar levels of partial protection against cutaneous and disseminated syphilitic infection and elevated antibody levels occurred in challenged normal guinea pigs passively immunized with lymphocytes from T. pallidum-infected TXB donors. In contrast, the capacity of the TXB host to be protected against a lethal infection with the unrelated intracellular protozoan parasite Toxoplasma gondii was greatly impaired unless it received an intravenous infusion of normal syngeneic thymocytes. These seemingly paradoxical results are explained primarily in terms of a residual T-helper-cell population in the TXB guinea pig which is large and competent enough to generate antisyphilis, but not anti-Toxoplasma, immunity.

Transfer of resistance to syphilitic infection from maternal to newborn guinea pigs

Partial protection against cutaneous syphilis and dissemination of treponemes to the regional lymph nodes was achievable in Treponema pallidum-infected neonatal strain 2 guinea pigs having syphilis-immune mothers. This evidence for enhanced resistance was associated with the natural transfer of antitreponemal, maternally derived immunoglobulins before parturition.