Role of CD28 in the Generation of Effector and Memory Responses Required for Resistance to Toxoplasma gondii

CD28 deficient (CD28−/−) mice were used to study the role of costimulation in the T cell-mediated, IFN-γ-dependent mechanism of resistance to Toxoplasma gondii. These mice were resistant to infection with the ME49 strain of T. gondii. Analysis of the immune response of acutely infected CD28−/− mice revealed that IL-12 was required for T cell production of IFN-γ and this was independent of the CD40/CD40 ligand interaction. A similar mechanism of IL-12-dependent, CD28/B7 independent production of IFN-γ by T cells was also observed in wild-type mice. Interestingly, although chronically infected wild-type mice were resistant to rechallenge with the virulent RH strain of T. gondii, chronically infected CD28−/− mice were susceptible to rechallenge with the RH strain. This deficiency in the protective memory response by CD28−/− mice correlated with a lack of IL-2 and IFN-γ in recall responses and reduced numbers of CD4+ T cells expressing a memory phenotype. Together, our findings demonstrate that CD28 is not required for the development of a protective T cell response to T. gondii, but CD28 is required for an optimal secondary immune response.

B7 Costimulatory Requirements of T Cells at an Inflammatory Site

The requirement for T cell costimulation at sites of infection and inflammation is unresolved. Herpes stromal keratitis (HSK) is a CD4+ T cell-regulated inflammatory response to herpes simplex virus type 1 infection of the cornea. Our findings suggest that susceptibility to HSK is determined by the microenvironment of the infected cornea. The cornea is normally devoid of Langerhans cells (LC), but these APC are present in the surrounding conjunctiva, and migrate into the cornea following infection. The costimulatory molecule B7-2 was constitutively expressed on LC in conjunctiva, but B7-1 was not detectable until 3 days postinfection. LC were the only cells in the cornea that expressed B7-1 through 7 days postinfection. B7-1 was expressed on some, but not all, migrating LC, suggesting that LC migration and B7-1 expression can be independently regulated. The early LC migration and B7-1 expression was independent of T cells, but T cells were required for the massive accumulation of B7-1+ LC in the cornea at the onset of inflammation. Local inhibition of B7-1 function within the infected cornea prevented HSK. Locally blocking B7-2 function did not reduce HSK incidence, but markedly reduce HSK severity. This is the first direct demonstration that naturally expressed B7 is required within an inflammatory site.

Dendritic Cells Are Required for the Development of Chronic Eosinophilic Airway Inflammation in Response to Inhaled Antigen in Sensitized Mice

Asthma is characterized by chronic eosinophilic inflammation of the airways, and allergen-specific Th2 lymphocytes are thought to play a major role in the development and maintenance of this type of inflammation in allergic asthma. It is generally accepted that airway dendritic cells (DC) are essential for stimulating naive T cells in a primary immune response to inhaled Ag and for the development of allergic sensitization. We have examined the role of airway DC in stimulating memory T cells in a secondary response to inhaled Ag and the subsequent development of chronic airway inflammation. In our mouse model of asthma, OVA aerosol challenge in OVA-sensitized mice leads to CD4-dependent peribronchial and perivascular eosinophilic inflammation, lung Th2 cytokine production, and systemic IgE production. We have used conditional depletion of airway DC by treatment of thymidine kinase-transgenic mice with the antiviral drug ganciclovir to deplete DC during the secondary exposure to OVA. In sensitized thymidine kinase-transgenic mice, a significant decrease in the number of bronchoalveolar CD4 and CD8 T lymphocytes and B lymphocytes was seen after ganciclovir treatment. In addition, Th2 cytokine-associated eosinophilic airway inflammation was almost completely suppressed. These studies demonstrate for the first time that the DC is essential for presenting inhaled Ag to previously primed Th2 cells in the lung, leading to chronic eosinophilic airway inflammation. Altering the function of airway DC may therefore be an important target for new anti-asthma therapy.

IL-12 Reverses Established Antigen-Specific Tolerance of Contact Sensitivity by Affecting Costimulatory Molecules B7-1 (CD80) and B7-2 (CD86)

Cutaneous painting with reactive haptens induces contact sensitivity (CS) responses that are in vivo examples of T cell immunity. In contrast, high dose i.v. administration of the hapten can induce tolerance. We investigated the effect of IL-12 on reversal of this tolerance and attempted to determine in vitro the mechanism of this reversing effect by measuring proliferation and IFN-γ production by CS effector T cells stimulated with hapten-conjugated APC, and we also measured CS ear swelling in vivo. The in vitro responses of T cells to hapten-APC became absent in tolerized mice, paralleling impaired in vivo CS responses. Addition of IL-12 to cultures manifesting this fully established in vitro tolerance completely restored impaired responses of tolerized T cells. The reversing effects of IL-12 were not blocked by anti-IFN-γ mAb, but were blocked by mAbs against B7-1, more strongly by anti-B7-2, and by both Abs together. Additional in vivo ear-swelling response experiments confirmed the reversing effects of IL-12 on established tolerance. To examine whether the IL-12 effect depended on stimulation of IFN-γ, we directly injected IFN-γ into tolerized mice. This partially mimicked but did not fully reconstitute the effects of IL-12. In summary, IL-12 abrogation of established tolerance of CS may have been partially due to endogenous production of IFN-γ, but appeared mainly due to direct activation of the tolerized T cells by affecting signaling through costimulatory molecules B7-1 and B7-2.