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

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.

Identification of T cell epitopes within a 23-kD antigen (P24) of Toxoplasma gondii

Among the potentially vaccinating antigens, the products excreted/secreted by the parasite T. gondii have been demonstrated to be excellent candidates. The molecular cloning of one of these antigens (P24) present in excreted/secreted antigens (ESA) has recently been carried out in our laboratory. The recombinant antigen P24 corresponds to a native molecule of 23 kD. We were interested in determining the main epitopes of the P24 antigen eliciting a T lymphocyte response using synthetic peptides derived from the primary structure of P24. Five peptides: 64-79, 88-109, 170-193, 194-208 and 231-250 were synthesized according to their hydrophobicity, mobility and accessibility profiles. The presence of T lymphocyte epitopes in these peptides has been examined in the rat model. The determination of T cell epitopes was carried out using T lymphocytes from infected rats, and from ESA and P24 expression vaccine virus immunized rats. The results showed that the stimulation of T cells with these peptides varied according to the period after Toxoplasma infection. The main T cell stimulation was obtained with the 88-109, 170-193 and 194-208 peptides. When Fisher rats were immunized with ESA, a most significant stimulation was achieved with the 170-193 and 194-208 peptides. In addition, T lymphocytes primed with P24 expressed vaccine virus immunization were more stimulated with the 88-109 and the 194-208 peptides. This study showed that P24-derived peptide-specific T cells were elicited in the three experimental situations, although no antibody response against the 23-kD native antigen was evidenced in the Fisher rat model. However, the native antigen (presented by irradiated parasites) can induce a proliferative response of the 170-193 peptide-specific T lymphocytes, confirming that this peptide contains an important T cell epitope. The adoptive transfer into athymic rats of T helper cells recovered from 170-193 peptide-immunized Fisher rat conferred a significant protection to infected nude rats despite the fact that no antibody production was observed.

Cyclophosphamide-sensitive activity of suppressor T cells during treponemal infection

When hamsters were infected with Treponema pallidum subspecies endemicum, the composition and activity of the cellular immune components were markedly altered compared to those of sham-infected controls. A population of suppressor T cells (Ts) developed that diminished the ability of the macrophage (M phi) to perform C3b receptor-mediated ingestion (C3bMI) of erythrocytes coated with antibody and complement. Using cyclophosphamide (CY) we examined node and peritoneal cells to determine their role in regulating M phi activity during this infection. In vitro the node and peritoneal T cells from treponemal-infected/CY-treated animals showed considerably less suppressive activity than treponemal-infected/untreated T cells when co-cultured with M phi from infected animals. This response was greater with node T cells compared to peritoneal T cells. Moreover, a quantitative analysis of the mononuclear leucocyte populations from each of these regions showed that CY-treated/uninfected animals had a decreased percentage of node T cells. Despite this reduction of node T cells, peritoneal T-cell populations were only minimally reduced. However, treponemal-infected hamsters concomitantly treated with CY had a significant reduction in the T-cell percentages in both compartments. These results imply that, during this infection, most Ts generated in the node remain there although some are dispersed to supplementary regions. Thus, the development of a suppressor system that effects M phi function may be one way in which treponemes escape total elimination by the host.