Mechanisms for recognition of tumor antigens and mediation of anti-tumor effect by noncytolytic Lyt-2+ T cell subset

CD8+ lymphocytes but not B lymphocytes are required for protection against chronic hepatitis E virus infection in chickens

Hepatitis E is an important global disease, causing outbreaks of acute hepatitis in many developing countries and sporadic cases in industrialized countries. Hepatitis E virus (HEV) infection typically causes self-limiting acute hepatitis but can also progress to chronic disease in immunocompromised individuals. The immune response necessary for the prevention of chronic infection is T cell-dependent; however, the arm of cellular immunity responsible for this protection is not currently known. To investigate the contribution of humoral immunity in control of HEV infection and prevention of chronicity, we experimentally infected 20 wild-type (WT) and 18 immunoglobulin knockout (JH-KO) chickens with a chicken strain of HEV (avian HEV). Four weeks postinfection (wpi) with avian HEV, JH-KO chickens were unable to elicit anti-HEV antibody but had statistically significantly lower liver lesion scores than the WT chickens. At 16 wpi, viral RNA in fecal material and liver, and severe liver lesions were undetectable in both groups. To determine the role of cytotoxic lymphocytes in the prevention of chronicity, we infected 20 WT and 20 cyclosporine and CD8⁺ antibody-treated chickens with the same strain of avian HEV. The CD8 ⁺ lymphocyte-depleted, HEV-infected chickens had higher incidences of prolonged fecal viral shedding and statistically significantly higher liver lesion scores than the untreated, HEV-infected birds at 16 wpi. The results indicate that CD8 ⁺ lymphocytes are required for viral clearance and reduction of liver lesions in HEV infection while antibodies are not necessary for viral clearance but may contribute to the development of liver lesions in acute HEV infection.

Characterization of extrathymic T cells of chickens

The function of CD4+ T cells in antibody production was examined by using T cell subset-depleted chickens. CD4- and CD8-depleted chickens, established by the combination of thymectomy and injection of T cell subset-specific monoclonal antibodies, were immunized with sheep red blood cells (SRBC). Titers of anti-SRBC antibody produced in CD4-depleted chickens were lower than those in control chickens, while no difference in the antibody production was observed between CD8-depleted and control chickens. In chickens depleted of both CD4+ and CD8+ T cells, the recovery of T cells in the periphery was demonstrated starting 3 weeks after T cell depletion. Those T cells recovered in the periphery predominantly expressed CD4 molecules. Although low titers of antibody against SRBC were detected in chickens depleted of both CD4+ and CD8+ T cells, an increase of anti-SRBC antibody production was coincidentally observed with the recovery of CD4+ T cells in the periphery. These results suggest that CD4+ T cells could differentiate in extrathymic environments in chickens, and have a helper function in antibody production similar to that of intrathymic T cells. These extrathymic T cells, however, showed a lower proliferative response to concanavalin A than intrathymic T cells, suggesting that these extrathymic T cells may have some properties distinct from intrathymic T cells.

Effector phenotypes and mechanisms of antitumor immune reactivity of tumor-immunized and tumor-bearing mice in two syngeneic tumors

By using two different syngeneic tumors, Meth A sarcoma and RL male 1 lymphoma of BALB/c origin, the present study was designed to investigate the subset(s) of T cells mediating in vivo antitumor immune responses and some of the effector mechanisms of in vivo protective immunity in BALB/c mice immunized against tumor or bearing tumor. Spleen cells from the mice immunized against Meth A tumor or bearing Meth A tumor inhibited the growth of Meth A tumor in the Winn assay. In the Meth A-immunized mice, L3T4+ (CD4+) cells played a major role in mediating the inhibitory activity against Meth A tumor growth, whereas in the Meth A-bearing mice, the antitumor protective immunity was mediated by both L3T4+ and Lyt-2+ (CD8+) cells. Spleen cells from the Meth A-immunized or Meth A-bearing mice were not able to generate cytotoxic T lymphocytes (CTL) directed against Meth A tumor after the in vitro restimulation of spleen cells with mitomycin C (MMC)-treated Meth A cells, while fresh spleen cells from the Meth A-immunized or Meth A-bearing mice were able to induce the strong delayed-type hypersensitivity (DTH) responses to Meth A tumor. The DTH response to Meth A tumor was mediated by L3T4+ cells in the Meth A-immunized mice and by both L3T4+ and Lyt-2+ cells in the Meth A-bearing mice. In the similar experiments performed in the RL male 1 lymphoma, the antitumor activity in spleen cells from the RL male 1-immunized or RL male 1-bearing mice depended on Lyt-2+ but not L3T4+ cells in the Winn assay. When spleen cells from the RL male 1-immunized or RL male 1-bearing mice were cultured with MMC-treated RL male 1 cells for 5 days, an appreciable CTL response to RL male 1 tumor was induced. These results suggest that the nature of tumor and/or tumor antigens determines which T cell subset is required to exhibit the protective immunity against tumor and thus the different effector mechanisms could be induced in the different tumor models. Furthermore, these data support the conclusion that antitumor T cell responses are affected by the immune state of host to tumor.

Helper strategy in tumor immunology: expansion of helper lymphocytes and utilization of helper lymphokines for experimental and clinical immunotherapy

Two main kinds of immune strategy are possible against neoplasia. The first potentiates a selected effector arm. In vitro culture with exogenous interleukin-2 (IL-2) increases the activity of natural killer cells and leads to the expansion of T cytotoxic lymphocytes. Systemic reinfusion of both of these cells with high doses of IL-2 mediates the regression of a variety of murine and human tumors. In an alternative strategy, a few regulatory lymphocytes turn on immune reactivity by triggering a cascade of interconnected effector functions. The efficacy of this strategy rests on the repertoire of effector mechanisms moved to action. An effective immunoregulatory maneuver is the addition of helper determinants on the surface of tumor cells. Its power can be further increased by the pre-induction of helper T lymphocytes specific to the helper determinants. This approach can be achieved in mice by coupling muramyl dipeptides to tumor cells, along with eliciting T lymphocytes specifically reactive to Bacillus Calmette-Guerin. Noncytotoxic T helper lymphocytes produce factors which recruit nonspecific (macrophages) as well as specific (cytolytic T lymphocytes) anti-tumor attacking cells. In this way protection can be afforded against primary tumors and metastases, as well as leukemia cells. As the activity of helper lymphocytes rests mostly on lymphokine release, the use of molecularly defined lymphokines mimicking T-helper functions has also been attempted. In a few experimental models, the association of low doses of IL-2 with non-reactive lymphocytes from tumor-bearing mice promotes an effective anti-tumor reaction in the host. Moreover, the combination of distinct lymphokines can also build a molecularly defined helper system able to activate in sequence non-specific and specific anti-tumor reactions in vivo. Trials intended to evaluate the clinical impact of these helper approaches in the management of human tumors are being started or are already under way.

Selective suppression of the generation of anti-tumor L3T4+ but not of Lyt-2+ T cell-mediated immunity in the tumor-bearing state

C3H/He mice hyperimmune against syngeneic MH134 hepatoma were prepared by intradermal (id) inoculation of viable tumor cells followed by surgical resection of the tumor and by repeated id challenges with viable tumor cells. Winn assays performed utilizing spleen cells from these mice have revealed that both Lyt-2+ and L3T4+ T cell subsets from MH134-hyperimmune mice produced complete tumor protection. The in vivo tumor-neutralizing activity was also found in spleen cells from tumor-bearing mice at various times after id implantation of MH134 tumor cells. However, in contrast to comparable tumor-neutralization by Lyt-2+ and L3T4+ T subsets from hyperimmune mice, only the Lyt-2+ T cell subset from tumor-bearing mice was capable of mediating the in vivo protective immunity. L3T4+ T cell-mediated immunity was not detectable in the tumor-bearing state irrespective of the length of the sensitization period with a primary growing tumor, but emerged in the mice which resisted the first tumor challenge after the resection of the primary tumor. These results indicate that the emergence of L3T4+ T cell-mediated anti-tumor immunity is stage-dependent and the Lyt-2+ T cells represent the main functional subset in the tumor-bearing state, although both subsets of T cells are potentially capable of effecting anti-tumor in vivo immunity. The results are discussed in relation to the selective suppression of the L3T4+ but not of Lyt-2+ T cell function in the tumor-bearing state.

The mode of recognition of tumor antigens by noncytolytic-type antitumor T cells: role of antigen-presenting cells and their surface class I and class II H-2 molecules

The present study investigated the role of antigen-presenting cells (APC) in the activation of noncytolytic T cells against tumor antigens. The noncytolytic-type T cells exerted their antitumor effect by producing gamma-interferon (IFN-gamma) and by activating macrophages as the ultimate effectors. The production of IFN-gamma by these noncytolytic T cells following the stimulation with tumor cells required the participation of Ia+ APC, since the depletion of APC from cultures of tumor-immunized spleen cells resulted in almost complete inhibition of the IFN-gamma production. Both L3T4+ and Lyt-2+ subsets of T cells were capable of producing IFN-gamma, and the requirement of APC for the production of IFN-gamma was the case irrespective of whether noncytolytic T cells were of L3T4+ or Lyt-2+ phenotype. More importantly, it was demonstrated that the production of IFN-gamma by L3T4+ and Lyt-2+ T cells was inhibited by addition of the respective anti-class II and anti-class I H-2 antibody to cultures. These results indicate that antitumor L3T4+ or Lyt-2+ noncytolytic T cells are activated for the IFN-gamma production by recognizing tumor antigens in the context of self-class II or -class I H-2 molecules on APC.