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NARRATOR:Listen to part of a lecture in a biology class.
FEMALE PROFESSOR:So that's the overview of the human immune system.But we have a few minutes left. Any questions? George?
MALE STUDENT:Yes. You talked about T cells..."naive T cells"...Can you go over that part again, and also...why do we call'em that, anyway?
FEMALE PROFESSOR:All right. They're-they're known as T cells because they develop in the thymus...
MALE STUDENT:The what?
FEMALE PROFESSOR:Thymus...that's T-H-Y-M-U-S. It's a small organ in the body.Anyway, that's why we call'em that—they come from the thymus.And T cells are a part of the body's immune system.They can recognize and eliminate cells from outside the body that might cause disease.
FEMALE STUDENT:But why "naive"?I mean, we might call people naive if they don't have enough experience to know about...the dangers of the world.But how can you call a cell "naive?"
Well, when this type of immune cell encounters a cell from outside the body...like maybe a bacterium...it interacts with that bacterium and learns to recognize it, so whenever the immune cell runs into that kind of bacterium in the future, it'll attack and kill it.At that point we call it a "memory T cell" because it's learned to recognize a protein marker that identifies this particular kind of bacterium.But before it's learned to recognize any particular protein from outside the body, we'd call it "naive." OK?
Yeah, I get it.
There's a lot of biochemistry involved...that we'll get into in the next lecture.But your question reminds me about a study that some of my colleagues are doing. It relates to caloric restriction.
Caloric? Like calories in the food we eat?
Exactly. We're talking about the sugars, carbohydrates, fats...that our bodies burn to get energy...which we measure in calories.OK, let's back up a little.
Back in the 1930s, a nutritionist at Cornell University put mice on a severely restricted diet.He gave each mouse in one group 30 percent less food...or, more precisely, 30 percent fewer calories than the mice in the other group...which ate a normal amount.And the result?The underfed mice lived much longer than the normally fed ones.
Wow, does that just go for mice?
Apparently not.Similar results have come from experiments on other animals-from roundworms to... most recently, rhesus monkeys.These monkeys-two groups of'em-were given all the vitamins and minerals and other nutrients they needed, except that one group got thirty percent fewer calories.And now, after thirty years or so-about an average lifetime for a monkey-it's clear that the monkeys that have been on the calorie-restricted diet are doing a lot better than the ones on what we'd consider a normal healthful diet.Like, in terms of blood pressure and lots of other measures, the calorie-restricted monkeys are much healthier, and they just look and act younger than the monkeys in the "normal-diet" group.And, as a group, they're living longer.
Interesting,...but what's the connection?
Oh, with the immune system?Well, it's been shown that the immune system becomes much less effective as animals age-that's true in humans too.We think those naive T cells just get used up-I mean it's not like the body's always making lots of new ones.And over the course of a lifetime, as T cells encounter more and more strange bacteria or whatever, the naive T cells get turned into memory T cells.So, later on in life, there are fewer and fewer of these naive T cells left to deal with any new disease-causing organisms that might attack.Which means less immunity, and the animal-or person-is more likely to get sick.
But caloric restriction... it kinda shocks the system, and one result is...well, those monkeys on the calorie-restricted diet had lots more naive T cells left than you'd expect in monkeys that old.The expected drop in naive T cells-apparently, the shock of getting thirty percent fewer calories really slows that down.And after many years, with so many more naive T cells still in reserve, these monkeys are a lot better at fighting off new infections than normally fed monkeys of the same...advanced age.
And that's why they live longer?Well, it's gotta be one reason.This is all pretty complex, though...with lots of details yet to be worked out.
But are results the same for humans?
Hard to say.A good study would take decades, and it's not easy finding people who'd want to take part.Would you?
And eat thirty percent less?That'd be tough.
You bet it would.
旁白：请听生物课上的部分内容。
教授：所以那就是对人类免疫系统的概观。不过我们还剩下几分钟时间。有什么问题吗？George？
学生：有，你讲到过T细胞（胸腺依赖型细胞），幼稚型T细胞。你能再过一遍那部分吗？还有，我们到底为什么那么叫它们呢？
教授：好的。它们被叫做T细胞是因为它们是在胸腺中发育的。
学生：什么？
教授：胸腺，拼写是T-H-Y-M-U-S。它是身体中的一个小器官。反正这就是我们这么叫它们的原因，它们来自胸腺。而且T细胞是身体免疫系统的一部分。它们可以辨认出并消灭来自身体之外可能会带来疾病的细胞。
学生：但是为什么说它们幼稚呢？我是说，我们会说人幼稚，如果他们没有足够的经验了解。。。世界上的危险的话。但是你怎么能说一个细胞幼稚呢？
当这种类型的免疫细胞遇到了来自身体之外的细胞。。。比如一个细菌，它会和那个细菌互动并学着辨认它，所以无论将来什么时候这个免疫细胞遇到了那种细菌，它都会攻击并杀掉这种细菌。在那个阶段，我们称它为记忆性T细胞，因为它学会了辨认代表着这种细菌的蛋白标志物。但是在它学会辨认任何来自身体之外的具体蛋白质之前，我们称它们为幼稚型，懂了吗？
嗯，我懂了。
这其中涉及了很多生物化学知识，我们下堂课会讲到的。但是你的问题让我想起了我的一些同事正在做的一项研究，它和卡路里限制有关。
卡路里的？就像我们吃的食物中的卡路里一样？
没错！我们讲的是糖分、碳水化合物、脂肪，我们身体燃烧以获取能量的。。。我们用卡路里来衡量。好了，我们往回讲一点。
在过去二十世纪三十年代的时候，一位Cornell大学的营养学家给老鼠制定了一个严格限制的食谱。他少给了一组里的每只老鼠30%的食物量，或者更确切地说，比另一组老鼠少30%的卡路里。。。正常食量的。结果是？食物不足的老鼠比正常喂养的老鼠活得久得多。
哇，这只适用于老鼠吗？
显然不是。用其他动物所做的实验都得出了相似的结果。。。从蛔虫到最近的猕猴。这些猴子被分成了两组，得到了它们所需的所有维他命和矿物质以及其他的营养物质，只不过一组少得到了30%的卡路里。在30年左右后，也就是大约一只猴子的平均寿命，很明显看到那些按照卡路里限制食谱进食的猴子比那些按照我们认为是普通的健康食谱进食的猴子活得好得多。在血压和很多其他标准中，卡路里限制的猴子要健康得多，而且它们比普通饮食组的猴子看起来更年轻，行为上也更年轻。它们一整个小组活的时间都更长。
很有趣，但是有什么联系呢？
哦，和免疫系统有什么联系吗？研究显示，随着动物变老，它们的免疫系统会变得不那么有效—这在人类身上也是一样。我们认为那些幼稚型T细胞被耗尽了，我是说，身体不是一直都会制造很多新T型细胞的。在一生中，随着T细胞遇到越来越多的陌生细菌或者随便其他什么细胞，幼稚型T细胞会变成记忆性T细胞。所以在生命的晚年阶段，剩下的对付新疾病的这些幼稚型T细胞越来越少，会产生可能会引发疾病的微生物。这就意味着免疫力下降，动物或人类生病的可能性就更大。
但是卡路里限制，它有点惊吓住了整个系统，一个结果就是，那些使用了卡路里限制食谱的猴子会有更多幼稚型T细胞剩下，这个数量远比我们在年纪那么大的猴子身上预期看到的要多。预计的幼稚型T细胞的减少，显然减少30%卡路里受到的惊吓真的减缓了这个过程。很多年后，仍有着那么多幼稚型T细胞，这些猴子在对抗新感染方面比同样年纪的普通喂养的猴子表现要好得多。
这就是为什么它们活得更久吗？这一定是其中一个原因。不过这一切都非常复杂，还有很多细节有待查明。
但是这些结果对人类来说也是一样的吗？
很难说。认真研究要花十几年时间。而且找到愿意参与实验的人不太容易。你会愿意参加吗？
要少吃30%？那太难了。
你肯定能做到的。
如果对题目有疑问，欢迎来提出你的问题，热心的小伙伴会帮你解答。