Toshiki Yajima

During the course of acute infection with an intracellular pathogen, Ag-specific T cells proliferate in the expansion phase, and then most of the T cells die by apoptosis in the following contraction phase, but the few that survive become memory cells and persist for a long period of time. Although IL-15 is known to play an important role in long-term maintenance of memory CD8⁺ T cells, the potential roles of IL-15 in CD8⁺ T cell contraction are not known. Using an adoptive transfer system of OT-I cells expressing OVA_257–264/K^b-specific TCR into control, IL-15 knockout (KO) and IL-15 transgenic (Tg) mice followed by challenge with recombinant Listeria monocytogenes expressing OVA, we found that the survival of CD44⁺CD62L⁻CD127⁻ effector OT-I cells during the contraction phase is critically dependent on IL-15. In correlation with the expression level of Bcl-2 in OT-I cells, the number of OT-I cells was markedly reduced in IL-15 KO mice but remained at a high level in IL-15 Tg mice during the contraction phase, compared with control mice. In vivo administration of rIL-15 during the contraction phase in IL-15 KO mice inhibited the contraction of effector OT-I cells accompanied by up-regulation of Bcl-2 expression. Furthermore, enforced expression of Bcl-2 protected the majority of effector OT-I cells from death in IL-15 KO mice after infection. These results suggest that IL-15 plays a critical role in protecting effector CD8⁺ T cells from apoptosis during the contraction phase following a microbial infection via inducing antiapoptotic molecules.

We previously found that an extracellular polysaccharide, AC‐1, produced by Acetobacter polysaccharogenes composed of (1,4)‐β‐D‐glucan with branches of glucosyl residues showed a strong activity to induce production of interleukin (IL)‐12 p40 and tumor necrosis factor‐α by macrophage cell lines in vitro via Toll‐like receptor‐4 signaling. In the present study, we examined the effects of oral administration of AC‐1 on protection against 2 types of murine B16 melanoma lines, major histocompatibility complex (MHC) class I‐negative B16L and MHC class I gene‐transfected B16K^b cells. Mice were inoculated subcutaneously with B16L or B16K^b cells on day 0 and administrated intragastrically with AC‐1 or PBS once every 5 days from 1 day before tumor inoculation. The tumor growth was severely retarded in AC‐1‐treated mice after subcutaneous inoculation with B16L or B16K^b cells. The AC‐1‐treated mice showed augmented natural killer (NK) cell activity against B16L cells, and in vivo depletion of NK cells by antiasialoGM1 antibody (Ab) treatment abrogated the antitumor activity in AC‐1‐treated mice. On the other hand, AC‐1‐treated mice inoculated with B16K^b cells developed a significantly higher level of cytotoxic T‐lymphocyte response against B16K^b cells, and in vivo depletion of CD8⁺ T cells by anti‐CD8 mAb treatment abrogated the antitumor activity. Thus, AC‐1 augmented antitumor activity against different tumors via augmentation of different antitumor mechanisms. These results suggest a possible prophylactic application of AC‐1 for human neoplasms irrespective of expression levels of their MHC class I molecules.

We previously found that AC-1, an extracellular polysaccharide, produced byAcetobacter xylinumand composed of (1,4)-β-d-glucan with branches of glucosyl residues, showed a strong activity to induce production of interleukin-12 (IL-12) p40 and tumor necrosis factor alpha by macrophages in vitro via Toll-like receptor 4 (TLR-4) signaling. In the present study, we examined the effect of oral administration of AC-1 on protective immunity againstListeria monocytogenes. Mice were given AC-1 or phosphate-buffered saline (PBS) intragastrically 2 days before, on the day of, and 2 days after an intraperitoneal inoculation ofL. monocytogenes. The survival rate of AC-1-treated mice was significantly improved and bacterial growth in AC-1-treated mice was severely retarded compared to those of PBS-treated mice after infection withL. monocytogenes. IL-12 p40 levels in serum and magnitudes of CD4⁺Th1 and CD8⁺Tc1 responses againstListeriaantigen were significantly higher in AC-1-treated mice than in PBS-treated mice. The effect of AC-1 on antilisterial activity was diminished in C3H/HeJ mice carrying mutated TLR-4. Thus, AC-1, a potent IL-12 inducer through TLR-4, enhanced protective immunity againstL. monocytogenesvia augmentation of Th1 responses. These results suggest that infectious processes driven by intracellular microorganisms could be prevented to develop by the (1,4)-β-d-glucan.

Mice primed withMycobacterium bovisbacillus Calmette-Guérin (BCG) are highly sensitive to lipopolysaccharide (LPS)-induced liver injury and lethality. We found that interleukin-15 (IL-15) transgenic (Tg) mice primed with BCG were more susceptible to LPS-induced liver injury than non-Tg mice. The numbers of CD44⁺CD8⁺T cells expressing intracellular gamma interferon (IFN-γ) significantly increased in the livers of BCG-primed IL-15 Tg mice after LPS injection, and the depletion of CD8⁺T cells from BCG-primed IL-15 Tg mice completely abolished the susceptibility to LPS-induced lethality. Liver T cells from BCG-primed IL-15 Tg mice produced IFN-γ in vitro in response to LPS, which was inhibited by the addition of anti-IL-12 monoclonal antibody (MAb). In vivo treatment with anti-IL-12 MAb inhibited the appearance of CD44⁺CD8⁺T cells expressing intracellular IFN-γ after LPS injection. These results suggest that the overexpression of IL-15 increases susceptibility to LPS-induced liver injury in BCG-primed mice via bystander activation of CD8⁺T cells.