Xenospecific CD8+ cytotoxic T lymphocyte generation: accessory function for CD4+ T cells and natural killer 1.1+ cells

Antagonistic effect of NK cells on alternatively activated monocytes: a contribution of NK cells to CTL generation

Natural killer (NK) cells fulfill essential accessory functions for the priming of antigen-specific cytotoxic T lymphocytes (CTLs). On the basis of a NKG2D-ligand-positive tumor model, we obtained results implicating NK-mediated regulatory as well as NK-mediated cytolytic activities in the initiation and persistence of CTL activity. Indeed, CD8(+) T-cell-dependent tumor rejection requires NK cell function in vivo, because tumors will progress both on depletion of NK cells or in the absence of optimal NK activity. Here we provide evidence that the absence of NK cells during subcutaneous tumor growth will abrogate generation of antitumor CTL responses and that this process can be linked to the expansion of alternatively activated monocytes. Indeed, our in vitro studies demonstrate that in splenic cultures from NK-deficient tumor-bearing mice, lack of type 1-associated cytokines correlates with the presence of type 2 (alternatively activated) monocytes and the production of type 2 cytokines. Furthermore, these type 2 monocyte-containing splenic adherent populations potently suppress subsequent memory CTL restimulation. We evaluated the role of NK lytic effector functions in the efficient switch of the immune system toward classical (type 1) activation by including differentially activated monocytic populations as targets in cytotoxicity assays. The results indicate that the accessory function of NK cells depends partially on the ability of activated NK cells to preferentially engage type 2 antigen-presenting cells. Thus, when the immune system tends to be type 2 oriented, NK cells can drive an efficient type 2 --> type 1 switch in the population of antigen-presenting cells to provide signaling for the generation of CTLs.

Without CD4 help, CD8 rejection of pig xenografts requires CD28 costimulation but not perforin killing

Although CD4 cells are major mediators in cellular rejection of fetal pig pancreas (FPP) in the mouse, rejection still occurs in the absence of CD4 cells, albeit with delayed kinetics. CD4 cell-independent mechanisms of cellular rejection are poorly understood. To investigate the involvement of CD8 T cells in FPP rejection and their activation requirements, we used mice transgenic for anti-CD4 Ab; this is the most complete model of CD4 cell deficiency. We showed that in such mice FPP was infiltrated with CD8 cells starting from 2 wk posttransplantation and FPP was eventually rejected 8 wk posttransplantation. Ab depletion of CD8 cells greatly improved the survival of FPP and reduced cell infiltration at the graft site. This suggests that CD8 cells can mediate the rejection of porcine xenografts in the absence of CD4 cells. This CD8-mediated rejection of FPP is independent of their perforin-mediated lytic function, as graft survival was not affected in mice deficient in perforin. The production of IFN-gamma and IL-5 by the graft infiltrates indicates that CD8 cells may act through cytokine-mediated mechanisms. Remarkably, in the absence of CD4 cells, lymphocyte infiltration at the graft site was absent in mice transgenic for CTLA4Ig such that the islet grafts flourished beyond 24 wk. In contrast, rejection was little affected by CD40 ligand deficiency. Therefore, we show that CD8 cells are activated to mediate FPP rejection independent of perforin and that this CD4-independent activation of CD8 cells critically depends on B7/CD28 costimulation.

Accessory function for NK1.1+ natural killer cells producing interferon-gamma in xenospecific cytotoxic T lymphocyte differentiation

BACKGROUND

We have previously demonstrated that xenospecific cytotoxic T lymphocyte (CTL) differentiation requires accessory function by NK1.1+ cells, yet the mechanism by which NK1.1+ cells support CTL generation had not been elucidated.

METHODS

An established model in which mice generate a strong local popliteal lymph node CTL response to footpad immunizations with human tumor cells was used. Mice depleted of NK1.1+ cells fail to mount a maximal xenospecific CD8+ CTL response. The xenospecific CTL response in anti-NK1.1 monoclonal antibody-depleted mice could be completely restored if mice were coinoculated with human tumor cells (the xenoantigen) and xenoantigen-stimulated syngeneic natural killer (NK) cells from wild-type or perforin-deficient mice. By contrast, NK1.1+ cells from interferon-gamma-deficient mice did not restore the maturation of xenospecific CTL in anti-NK1.1 monoclonal antibody-treated mice. Depletion of NK1.1+ cells in vivo from both wild-type and Jalpha281-deficient mice (which lack Valpha14 NK1.1+ T cells) abrogated the generation of xenospecific CTL, however, untreated Jalpha281-deficient mice mounted a normal xenogeneic response.

CONCLUSIONS

These data indicate that local NK cell production of interferon-gamma at the site of challenge is an important stimulus for generating xenospecific CTL in local draining lymph nodes and that Valpha14 NK T cells play little or no regulatory function in this response.