EFFECT OF CORTISONE ON THE COURSE OF EXPERIMENTAL SYPHILIS IN THE GUINEA-PIG. I. EFFECT OF PREVIOUSLY-ADMINISTERED CORTISONE ON GUINEA-PIGS INFECTED WITH TREPONEMA PALLIDUM INTRADERMALLY, INTRATESTICULARLY, AND INTRAVENOUSLY

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.

Humoral response in Treponema pallidum-infected guinea pigs: I. Antibody specificity

Young male inbred strain 2 guinea pigs were infected intradermally with 8 X 10(7) Treponema pallidum extracted from a rabbit orchitis, and 5 months later reinfected with 10(7) T. pallidum. Ninety percent of the animals developed symptomatic lesions after initial infection but none on challenge. Immunoblotting of sera obtained at intervals after infection or reinfection showed antibodies against T. pallidum antigen (TP), nonpathogenic treponemes--T. phagedenis biotype Reiter (TR), T. refringens strain Noguchi (TN), and T. vincentii (TV)--as well as normal rabbit serum (NRS) and normal rabbit testes extract (NRT). Antibodies reacting with TP were detected as early as 17 days (five polypeptides) and steadily rose (at 3 months 17 polypeptides were seen). Cross-reacting antibodies to TR, TN, TV, or rabbit proteins decreased within 3 to 5 months. After reinfection, the antibodies to NRS increased more sharply than the anti-treponemal antibodies. Adsorption with TR and NRS of sera obtained after infection or reinfection produced a reduction of antibodies to TP by 75-87%.

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.

Experimental syphilis: guinea pig model

Outbred Hartley male guinea pigs (300-400 g) were infected intradermally with various concentrations of Treponema pallidum Nichols strain in the pubic region. The median lethal dose (ID50) was approximately 10(5). The animals produced lesions visible by darkfield microscopy, treponemal antibodies (IgG only), and histopathological changes in the lymphoid organs. Though less susceptible to T pallidum infection than the rabbit when infected with a sufficient number of organisms, the guinea pig may be a useful model in experimental syphilis.