Susceptibility of Interleukin-2-Deficient Mice to Toxoplasma gondii Is Associated with a Defect in the Production of Gamma Interferon

Costimulation through the B7-CD28 interaction is an important second signal for T-cell activation, and previous studies have shown that CD28(−/−) mice infected with Toxoplasma gondii generate suboptimal CD4(+) T-cell responses, associated with a defect in production of the T-cell growth factor interleukin-2 (IL-2). To address the role of IL-2 in the expansion of T cells during toxoplasmosis, IL-2(−/−) mice were infected with T. gondii and their ability to generate a protective T-cell response was assessed. Although IL-2(−/−) mice produced normal levels of IL-12p40, they had reduced levels of gamma interferon (IFN-γ) in serum, had an increased parasite burden, and succumbed to infection with T. gondii within 20 days. Fluorescence-activated cell sorter analysis revealed that, although uninfected IL-2(−/−) mice had an increased number of activated T cells compared with uninfected IL-2(+/+) mice, following infection they were unable to further upregulate this population. Examination of the ability of splenocytes from uninfected and infected mice to produce IFN-γ revealed that IL-2(−/−) mice were hyporesponsive to stimulation with anti-CD3 or parasite antigen compared with wild-type mice, and the addition of IL-2 alone or in combination with IL-12 or stimulation with phorbol myristate acetate and ionomycin did not restore the production of IFN-γ. Together, these studies reveal that IL-2(−/−) mice are unable to generate a protective IFN-γ response following infection with T. gondii and suggest that IL-2(−/−) mice have an intrinsic defect in their ability to activate and expand IFN-γ-producing T cells required for resistance to T. gondii.

B7-CD28相互作用介导的共刺激信号是T细胞活化的关键第二信号。既往研究显示，感染刚地弓形虫（Toxoplasma gondii）的CD28基因敲除（CD28(−/−)）小鼠可产生亚最优的CD4阳性（CD4(+)）T细胞应答，且伴随T细胞生长因子白细胞介素2（interleukin-2, IL-2）产生缺陷。为探究IL-2在弓形虫病期间T细胞扩增中的作用，本研究对IL-2基因敲除（IL-2(−/−)）小鼠感染刚地弓形虫，并评估其产生保护性T细胞应答的能力。尽管IL-2基因敲除小鼠的白细胞介素12p40（interleukin-12p40, IL-12p40）水平正常，但其血清中γ干扰素（gamma interferon, IFN-γ）水平降低，寄生虫负荷升高，并在感染后20天内死于刚地弓形虫感染。荧光激活细胞分选仪（fluorescence-activated cell sorter, FACS）分析显示，未感染的IL-2基因敲除小鼠相较于未感染的IL-2基因野生型（IL-2(+/+)）小鼠，活化T细胞数量增多，但感染后无法进一步上调该细胞群的比例。对未感染及感染小鼠的脾细胞产生IFN-γ的能力进行检测后发现，相较于野生型小鼠，IL-2基因敲除小鼠在受到抗CD3刺激或寄生虫抗原刺激时呈低应答状态；且单独添加IL-2、或联合IL-12添加，亦或是用佛波醇肉豆蔻酸乙酸酯（phorbol myristate acetate, PMA）与离子霉素（ionomycin）刺激，均无法恢复其IFN-γ的产生。综上，本研究证实IL-2基因敲除小鼠在感染刚地弓形虫后无法产生保护性IFN-γ应答，并表明IL-2基因敲除小鼠存在固有缺陷，无法活化并扩增抵抗刚地弓形虫感染所需的产IFN-γ T细胞。