Immune system breakdown increases the risk of developing cancer
Adam Cresswell, The Australian
-
Having HIV increases the risk of developing 20 different types of cancer - not just the three types that have been associated with the virus to date, according to Australian research.
The findings, published last week in the UK journal The Lancet in the run-up to an international AIDS conference in Sydney, will have "huge implications'' for the future care of HIV-infected patients, according to an accompanying editorial in the journal.
But the study (2007;370:59-67) is also likely to change understanding of the role of the immune system in fighting cancer, and not just for HIV-infected patients.
The study, by professionals from the University of NSW, reviewed the results of 12 previous trials. Seven of those looked at cancer rates in a total of nearly half a million patients with HIV, while the five other studies examined cancer rates in 31,977 kidney transplant recipients.
Transplant recipients have to take drugs that suppress the immune system to prevent their bodies rejecting the transplanted organ, while people infected with HIV also have impaired immune function as a result of their infection, as the HIV virus particularly targets immune cells.
Previous attempts to examine the effect of immune system compromise on cancer rates have suffered from the relative scarcity of transplant patients - the main circumstance that led to impaired immunity before the AIDS era - which made it difficult to gather enough study subjects for the results to be statistically meaningful.
Previously, AIDS - the end-stage of HIV infection, caused by opportunistic infections overwhelming the body's run-down immune defences - has only been associated with three cancers: non-Hodgkin's lymphoma, cervical cancer and Kaposi's sarcoma.
But when the UNSW authors combined the previous study data, they found that rates of 20 out of the 28 cancers studied were significantly increased in both the transplant and HIV-infected groups.
"We believe that the striking similarity in patterns of increased cancer risk that we have shown indicates that immune deficiency is the probable explanation for the increased cancer risk," they wrote. "The increased rates of cancers we have found ... suggests a broader than previously appreciated role for the immune system in the prevention of cancers related to infection. If immune deficiency is associated with such a broad range of cancer types, then cancer is likely to become an increasingly important cause of morbidity in people with HIV/AIDS."
Lead author professor Andrew Grulich, head of the HIV epidemiology and prevention program at UNSW's National Centre in HIV Epidemiology and Clinical Research, said a strength of the study was that the two groups it compared had little in common other than their immune compromise.
Cancers that affected both groups at higher rates included Hodgkin's lymphoma - associated with the Epstein Barr virus - which is 11 times more likely in HIV-infected patients than in the general population, and four times more likely in transplant patients.
A range of other cancers associated with the human papillomavirus, such as cervical cancer, and cancers of the mouth, penis and anus, were also significantly more common across both groups.
"In other cancers, which are not linked with viruses, such as breast and prostate cancer, both groups had similar rates to the general population," Professor Grulich said. "This evidence suggests that immune deficiency is associated with risk of cancer, and this suggests we need to maintain people's immune systems at a higher level. That might mean putting HIV patients on anti-retroviral drugs earlier than is currently the case."
The study's results also suggest that CD4 counts - a measure of how many of one type of immune cell targeted by HIV are circulating in a patient's blood - is an imperfect measure of immune function.
CD4 counts are often used to gauge effectiveness of anti-HIV therapy, and when an infected patient should start using it. But previous studies have shown a poor association between CD4 counts at the point when a patient is diagnosed with AIDS and cancer rates, suggesting other immune systems are more important.
The Lancet devoted its entire July 6 issue to HIV, ahead of the International AIDS Society Conference, which is expected to attract 5000 delegates and will be held in Sydney from July 22 to 25.
-
