脳腫瘍免疫の特殊性に関する考察—T細胞サブセットの動態に関連した解析

ANALYSIS OF THE PECULIARITY OF IMMUNOLOGICAL RESPONSES TO EXPERIMENTAL BRAIN TUMOR

　抄録　20—methylcholanthrene誘発マウス（C 57 BL／6）可移植性神経膠芽腫203—gliomaを正常又は胸腺摘出（adult thymectomy,ATx）成熟マウスの脳内に移植し，各々の担腫瘍状態におけるT細胞subsetの動態をマウスT細胞の分化抗原（Lyt抗原）を用い解析した。更に，腫瘍増殖に及ぼすATxの影響をもとにして，脳腫瘍のT細胞に関連した免疫応答を皮下腫瘍の場合と比較する事により，その特殊性を検索した。その結果，脳腫瘍系でも，同系腫瘍で誘導されるキラーT細胞は，腫瘍抗原特異姓を有し，ATxに影響されない長命なLyt−1^—，2.3^＋細胞とそれに反し短命なLyt−1^＋．2.3^＋細胞の異なる細胞から構成されていた。しかし，後者は頭蓋内圧亢進により速やかに消失するため，宿生のキラーT細胞活性は移植後早期より，急減する事がわかった。Mitomycin-C （MMC）処理腫瘍細胞の皮下又は脳内移植に続く腫瘍生細胞（MMC非処理）の皮下又は脳内移植による腫瘍増殖の影響から，脳内の抗原認識および随伴免疫能が不完全である事がわかった。腫瘍増殖に及ぼすATxの影響として，脳腫瘍の場合，頭蓋内圧亢進の出現によりin vivoにおける生存日数への影響はみられなかったが，in vitroにおけるキラーT細胞活性への影響は皮下腫瘍の場合と同様の影響がみられた。

Immunological responses in relation to the ki-netics of killer T cell induction to intracranially transplanted syngeneic murine malignant glioma cells (20-methylcholanthrene-induced glioma, 203-glioma) were investigated in C57BL/6 mice in order to elucidate the peculiarity of immunological re-sponses to brain tumor.

Small dose of 203-glioma cells (1×10⁵) unable to be taken in subcutaneous inoculation was taken in intracranial inoculation. The killer T cell activity of splenic cells, which was specific for 203-glioma cells and almost completely elimina-ted by the treatment with anti-Thy-1 monoclonal antibody and complement, began to be severely impaired 2 weeks after intracranial inoculation concurrently with the increased intracranial pres-sure due to developing tumor growth. On the other hand, the killer T cell activity of splenic cells was maintained for over 4 weeks in mice inoculated with mitomycin-C treated 203-glioina cells.

Follwing subcutaneous inoculation (at the dose of 9×10⁵) of mitomycin-C treated 203-glioma cel-ls, viable 203-glioma cells were re-inoculated in-tracranially (1×10⁵, 5×10⁵) or subcutaneouslly (5×10⁵, 1×10⁶) 10 days later. In the former mice, each median survival time (MST) was 12.2 and 6.6 weeks and each tumor death rate in 8 weeks (TDR) was 44.4% and 100%. On the other hand in the latter mice tumor was not taken at the dose of 5×10⁵ 203-glioma cells and regressed 80 % at the dose of 1 x 106 203-glioma cells.

Following intracranial inoculation (at the dose of 5×10⁵) of mitomycin-C treated 203-glioma cells, viable 203-glioma cells were re-inoculated intracranially (1×10⁵, 5×10⁵) or subcutaneously (5×10⁵, 1×10⁶) 10 days later. In the former mice, each MST was 8.8 and 5.6 weeks and each TDR was 90% and 100%. On the other hand, in the latter mice tumor was regressed 75% at the dose of 5×10⁵ 203-glioma cells. MST at the dose of 1×10⁶ 203-glioma cells was 10.8 weeks. Each TDR was 0% and 25%. These results strongly suggested that the concomitant immunity in brain tumor was generated incompletely and partially.

The surface markers of killer T cells were chec-ked with the results that in intracranial tumor-bearing mice Lyt-l^-.2.3⁺ killer T cells were pre-dominant, whereas both Lyt-1^-.2.3⁺ and Lyt-1⁺. 2.3⁺ killer T cells were equally mediated in sub-cutaneous tumor. Lyt-1⁺. 2.3⁺ killer T cells were thought to disappear rapidly in the presence of increased intracranial pressure.

The effect of adult thymectomy on in vivo and in vitro responses to 203-glioma were also investigated in order to analyse the role of T cell subpopulation in brain tumor. Median survi-val time (MST) in adult mice thymectomized 3 and 10 weeks before intractanial inoculation of tumor cells was not shorter than in sham-thymec-tomized controls. On the other hand, the killer T cell activity of splenic cells was increased in mice thymectomized 3 weeks before tumor cell inocu-lation and decreased in mice 10 weeks before tu-mor cell inoculation. In mice tymectomized 3 and 10 weeks before tumor cell inoculation, Lyt-1⁺. 2.3⁺ killer T cells were not detected in intra-cranial tumor as well as in subcutaneous tumor, suggesting that the progenitors of Lyt-1⁺. 2.3⁺ killer T cells are short-lived cells in contrast to those of Lyt-1^-.2.3⁺ killer T cells which survive more than 10 weeks after adult thymectomy.

Accordingly, in intracranial tumor-bearing mice the recognition of tumor associated antigen and the generation of tumor specific killer T cells are partially impaired because of increased intracra-nial pressure in addition to the absence of the lymphatic pathway and the presence of the blood brain barrier