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The effects of canine transmissible venereal tumor on dendritic cells and the interactions between dendritic cells and nature killer cells
Canine transmissible venereal tumor, CTVT,progression phase, P phase,regression phase, Rphase,peripheral blood mononuclear cell, PBMC,monocyte-derived DC, Mo-DC,immature dendriric cells, iDC,mature dendritic cell, mDC,mixed lymphocyte reaction, MLR,
|Publication Year :||2006|
|Abstract:||腫瘤經常以樹突狀細胞（dendritic cell, DC) 為目標，進而對DC造成傷害而藉此逃避生物體免疫系統的辨識。在小鼠和人類許多的腫瘤研究中都已經發現腫瘤會造成DC的傷害。一般腫瘤會透過下述幾種機制來影響及改變DC的活性，例如抑制DC前驅物的分化、壓制其成熟化反應的發生、妨礙吞噬及表現抗原能力的表現，或者造成DC的死亡。腫瘤通常會利用上述一種或多種不同的機制來促進腫瘤細胞的生長。
在我們的研究中發現犬傳染性花柳性腫瘤 (canine transmissible venereal tumor, CTVT) 也會透過所有上述的機制對於單核球來源的DC（monocyte-derived DC, Mo-DC）造成嚴重的傷害。我們將正常犬與接種腫瘤後所得到的Mo-DC，利用流式細胞分析儀（flow cyto- meter）及real-time PCR，比較其表面抗原表現，包括CD1a、CD83，共同刺激因子（costimulatory factors，如CD40，CD80和CD86) 以及MHC class I與II分子。由實驗中發現無論是未成熟的DC （immature DC, iDC）或者是透過脂多醣（lipopolysaccride, LPS）刺激活化的DC（LPS-treated mature DC, mDC），其表面抗原的呈現，在腫瘤的影響下，無論是生長期（progression phase，P期）或消退期（regression phase，R期），都比正常犬低。除表面抗原呈現外，DC的功能也受到腫瘤嚴重的破壞。在P期時發現iDC吞噬dextran的活性明顯比正常犬為低。而在同種異體淋巴球反應（Allogeneic mixed lymphocyte reaction, MLR）中也發現mDC刺激淋巴球活化的能力在腫瘤的影響下明顯受到抑制。此外，在CTVT的作用下我們發現犬隻周邊血液單核球細胞（peripheral blood mononuclear cell, PBMC）中單核球（monocytes）的數量減少約40%。而且在單核球分化為iDC或者是每顆iDC轉化成mDC的比例中，腫瘤接種犬都明顯比正常犬為低。有趣的是，在R期，受到抑制的DC活性似乎大部分已恢復。無論是mDC的數量、表面抗原的表現、抗原吞噬能力及MLR的表現都有明顯恢復的現象。我們相信應該是有一種未知的機制在腫瘤進入R期時促使Mo-DC的功能逐漸回復。
再者，近年來根據小鼠及人類NK細胞（nature killer cell, NK cell）及DC的研究中，發現二者之間的互動對於免疫反應的進行有明顯的影響，但對於犬隻相關文獻探討較少。本研究中另一目的是針對犬隻NK細胞及DC間的交互作用進行研究。在觀察NK細胞的細胞毒殺能力中發現，無論是休止型或者是以IL-2（Interleukin 12）活化後的LAK（lymphokine- activated killer cell）細胞，在CTVT的影響下，細胞毒殺能力都明顯降低。雖然在R期NK細胞的毒殺能力較P期為佳，但仍然明顯受到抑制。觀察在CTVT的生長時可以發現，無論在P期或R期，mDC的CD80，CD83及CD86表現會明顯下降。雖然CTVT影響DC的成熟，但DC在與NK細胞共同培養的作用時發現，NK細胞卻能協助DC的成熟。最後觀察細胞激素（cytokine）的變化，本實驗中以單獨未受任何刺激的DC作為對照組，發現在DC與NK細胞共同培養時，產生大量IL-12與IL-18，此種細胞激素會活化NK細胞。當CTVT生長於P期與R期時，除了iDC與mDC本身分泌的細胞激素量就比正常少外，與NK細胞共同培養後，雖然mDC所分泌的IL-12、IL-18、IFN-γ與TNF-α會顯著上升，但總量仍較正常明顯為少。但在R期時IL-12、IL-18、IFN-γ與TNF-α的分泌量都明顯比P期為多。
Tumors often target dendritic cells (DCs) to evade host immune surveillance. DC injury has been reported in many tumors in rodents and humans. They exert systemic impact by altering the activities of DCs through the following mechanisms: inhibition of differentiation, suppression of maturation, impediment of functions, or killing of the cells. Tumors usually employ one or more of these mechanisms to facilitate their progressive growth.
We reported here that canine transmissible venereal tumor (CTVT) induced devastating effects on monocyte-derived DCs through all of the mechanisms mentioned above. Using flow cytometry and real-time RT-PCR, the expression of the surface markers of monocyte-derived DCs, including CD1a, CD83, costimulatory factors (CD40, CD80, and CD86), and MHC classes I and II were compared between normal dogs and canine transmissible venereal tumor (CTVT) dogs. It was found that the tested surface markers were all or mostly lower in concentrations in both immature DCs (iDCs) and LPS-treated mature DCs (mDCs) from tumor dogs during the progression (P) or the regression (R) phase than they were in normal dogs. The functions of the DCs were also seriously damaged by the tumor. The iDCs from dogs with P-phase tumors performed significantly lower endocytotic activity on dextran uptake than the normal dogs. Allogeneic mixed-lymphocyte reaction (MLR) was much lower in the mDCs from tumor dogs. In addition, the tumor decreased the number of monocytes in the peripheral blood by 40%. The efficiency of DC generation per monocyte or mDC generation per iDC was much lower in tumor dogs. Interestingly, during R phase, the inhibited DC activities were partially recovered, as evidenced by the elevated number of the mDCs and higher expression of the surface markers, plus the better dextran uptake and MLR activities. It is believed that the monocyte-derived DCs re-established by the host through an unknown mechanism contributed to the regression of the tumor.
We also found that the 48h cultured P phase CTVT supernatants containing protein components killed both monocytes and DCs by apoptosis. Very few investigation reported tumor-caused damage on monocyte-derived DCs with such a wide spectrum. It is concluded that CTVT killed the DCs through apoptosis, inhibited the differentiation of the cells from monocytes and from iDCs to mDCs, and restrained the functions of the DCs. These caused devastating damage to the DCs and enabled the tumor to evade host immunity. These findings provide further understanding of the tumor effects on the host immune system and are useful in developing strategies for cancer immune therapy.
In mouse and humans, the NK cells (nature killer cells, NK cells) and DC interactions play an important role in moderating innate immune responses. In dogs, this interaction has rarely been studied. Thus, another goal of this research was to understand the interactions between NK cells and DCs in dogs. The results showed that the growth of the CTVT significantly hampered the cytotoxicity of the resting and IL-2 activated killer cells. Although the cytotoxicity of the NK cells during R phase was improved, the suppression of the activity was still obvious. The mRNA expression of CD80, CD83, and CD86, the markers for DC maturation, was also lower during the growth of CTVT. These indicated that CTVT also inhibited the maturation of DCs. The NK-DC interaction studies showed that NK cells apparently helped the maturation of mDCs from iDCs in dogs. On the other hand, during normal NK-DC co-culturing in dogs, IL-12 and IL-18 were produced in high concentration that were much higher than those cultured alone. These cytokines were potent cytokines to activate NK cells. In addition, the results also provided evidence that CTVT inhibited the general production of the cytokines. Furthermore, although the production of IL-12, IL-18, IFN-g, and TNF-a were increased during NK-DC co-culturing from CTVT dogs, it was still significantly lower than normal dogs without CTVT. Similarly, R-phase dogs showed some recovery of cytokine production.
To summarize, the diversifying injury due to the growth of CTVT caused devastating damage to DCs, including the inhibition of differentiation from monocytes to iDCs and iDCs to mDCs, the suppression of DC maturation, the impairment of DC functions such as antigen uptake and MLR activities, and the apoptosis of monocytes and DCs. In addition, this tumor also suppressed the NK cytotoxicity and hampered the interactions between DC and NK cells. Thus, CTVT is an ideal cancer model for studying the complex interrelationships between tumor growth and host reactions.
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