Immune surveillance against cancer

From this introduction, it should be clear that powerful defenses exist within the cell (e.g., tumor suppressor proteins) to deal harshly with most wannabe cancer cells. Whether or not the immune system also plays a major role in protecting us against the majority of human cancers is not nearly so clear.

In mice which have been engineered so that one or more of the components of their immune system is defective, an increased incidence of lymphoma, leukemia, and virus-associated cancer is well documented. However, the evidence that mice with compromised immune systems experience an increase in solid tumors that do not involve a virus infection is not compelling. In one study, for example, 12 mutant mice that lacked the RAG-2 protein (which is required to assemble B and T cell receptors) were examined. These mice, which are without functional B and T cells, showed no signs of disease while they were alive. However, autopsies revealed that 11 of them had small intestinal tumors, and one had a small lung tumor. Eleven normal mice, housed under similar conditions, had no tumors. These results, although suggestive, beg the question: if B or T cells really are important for protection against solid tumors, why were almost all of these tumors in the intestines and not in the other places that humans commonly get tumors? These mice were raised in “pathogen-free” conditions, so the bacteria in their intestines were different from the bacteria present in the intestines of ordinary mice. Consequently, it is possible that under these conditions, the combination of an unnatural intestinal flora and the RAG-2 mutation predisposed these mice to intestinal tumors. Moreover, because there are significant differences between mouse and human immune systems, it is difficult to know which experiments with mice are relevant to human cancer.

In humans there is also strong evidence that a weakened immune system can increase one’s chances of getting blood-cell and virus-associated cancer. However, the published data do not convincingly support the contention that a compromised immune system leads to an increase in the most common of all human tumors: those that are not of blood-cell origin and which are not virus-associated. For example, one oft-quoted study showed that patients who received heart transplants, and who were immunosuppressed to avoid rejection of their transplanted organ, had rates of lung cancer that were 25 times that of the general population. However, individuals who received other types of transplants (e.g., kidneys), and who also were immunosuppressed, did not experience increased rates of lung cancer. Consequently, the increase in lung cancer in the heart transplant patients might simply reflect the fact that many of the patients in this study were cigarette smokers.

So does the immune system provide significant protection against cancer? To try to answer this question, let’s examine the roles which various immune system cells might play in cancer surveillance – keeping in mind that different kinds of cancer may be viewed very differently by these cells.

Дата добавления: 2015-06-04