December 14, 2016
In Canada and other high-income countries, the widespread availability of HIV treatment (ART) has made deaths due to AIDS-related complications significantly less common than in the 1980s and 1990s. In the current era, doctors are finding that issues generally unrelated to AIDS are affecting the health and survival of HIV-positive people who take ART. As an example, research teams in Canada, Denmark, France and the U.S. are finding that some HIV-positive people are at elevated risk for lung cancer. In part, this problem of elevated lung cancer risk arises from the high rates of smoking among HIV-positive people. However, researchers have suspected that there may be additional risk factors for lung cancer in this population.
Recently, a team of researchers at several clinics in the U.S. has moved closer to uncovering these additional risk factors. The researchers analysed health-related data collected from nearly 22,000 HIV-positive people who were monitored for about seven years. The team found that, as would be expected, smokers were at elevated risk for lung cancer. In addition, they found that people who had a low ratio of two types of immunological cells (CD4+ cells and CD8+ cells) and others who had a history of bacterial pneumonia were at elevated risk for lung cancer. Later in this bulletin we explore these two additional factors and how they could have affected the risk for lung cancer.
The research team collected data between January 1, 1998 and December 31, 2012. The researchers focused on 21,666 HIV-positive veterans, 277 of whom ultimately developed confirmed cases of lung cancer.
The vast majority (98%) of participants were male and had between two and four T-cell counts done each year that they were in the study (in addition to other tests).
Over the course of the study, 277 participants (1.3%) developed lung cancer.
According to the researchers, participants who developed lung cancer were statistically more likely to have the following features upon entering the study:
In arriving at these results, the researchers also took into account the following factors from participants' medical records:
Two important groups of T-cells involved in the immune response to germs and tumours are CD4+ and CD8+ cells. Monitoring the level of CD4+ cells in the blood has been a key part of assessing the health of the immune system in people with HIV and other immunological disorders. In untreated HIV infection, the number of CD4+ cells falls and the CD8+ cell count rises.
The ratio of CD4/CD8 cells is used as a crude way of assessing the overall health of the immune system. In healthy HIV-negative adults, the lower end of the normal range for a CD4/CD8 ratio is usually around 1.0. However, in HIV infection the CD4/CD8 ratio is often less than this and can fall even lower the longer this infection remains untreated. Once ART is initiated, the CD4+ count often rises, usually at a slow pace over several years, and the CD8+ count often decreases, also sometimes at a slow pace. One study in Italy with more than 3,000 HIV-positive participants found that over a period of five years the use of ART led to a slow but steady improvement in initially very abnormal CD4/CD8 ratios. However, after five years, only about 30% of ART users had a CD4/CD8 ratio of at least 1.0.
Note that not everyone responds to ART in the same manner. For instance, in a minority of cases, the CD4+ count rises after ART is initiated but the CD8+ count does not fall. The net result is that the ratio of these two cells is less than normal, even if the viral load becomes undetectable. A low CD4/CD8 ratio despite good adherence to ART (as shown by an undetectable viral load) is a signal that the immune system is dysfunctional, and if low ratios persist it may require further investigation by a specialist in infectious diseases or immunology.
The reasons for minor or no significant improvement in the CD4/CD8 ratio despite good adherence to ART over several years are not clear and may differ from one person to another. Some research suggests that the longer the time period between HIV infection and the initiation of ART, the more time HIV has to damage the immune system. A study in the UK and several other countries has found that among 573 people who had recently become HIV positive, 84% had abnormal CD4/CD8 ratios. Furthermore, the longer that HIV remained untreated, the less likely it was for participants' CD4/CD8 ratio to normalize after initiation of ART. Similar findings were seen in a study in Montreal with 84 participants who began early initiation of ART vs. 49 participants who started later.
Therefore, several research teams have suggested initiating ART as soon as possible after HIV infection to help prevent the CD4/CD8 ratio from becoming very abnormal and to make it more likely that the immune system can have a better response to ART.
Another potential factor that may be associated with a prolonged low CD4/CD8 ratio despite the use of ART could be older age. As a person ages, so does their immune system. People who become infected later in life and who then initiate ART may sustain more immunological injury than a younger person. A French study of 399 HIV-positive people monitored for 10 years found that participants who initiated ART when they were 40 years or older were less likely to eventually normalize their CD4/CD8 ratio than younger people.
In the same French study, researchers found that those participants who interrupted ART were more likely to have a low CD4/CD8 ratio.
Several teams of researchers, including ones based in Canada, have found evidence suggesting that the virus called CMV (cytomegalovirus; a member of the herpes virus family and relatively common among gay, bisexual and other men who have sex with men) may be linked to an abnormal CD4/CD8 ratio in some ART users. Research teams in France and the U.S. have also pointed a finger at CMV co-infection as a possible reason for persistent abnormal CD4/CD8 ratios despite the use of ART with suppressed viral loads.
The U.S. team studying lung cancer risk did not explore possible co-infection with CMV or other herpes viruses and their potential impact on the CD4/CD8 ratio.
Keep in mind that the complexity of the immune system is immense and scientists do not know everything about it.
The research team studying HIV-positive veterans found that having a prolonged low CD4/CD8 ratio (1.0 or less) was significantly associated with an increased risk for lung cancer.
Another finding was that participants with a history of bacterial pneumonia were also at elevated risk for lung cancer.
The U.S. team studying lung cancer risk found that a history of bacterial pneumonia was linked to an increased risk for lung cancer. The team is not certain why this relationship exists but thinks that the inflammation in the lungs triggered by bacterial pneumonia may in some cases incite the development of abnormal growth among lung cells. Ultimately, these abnormal cells may become pre-cancers and, in some people, cancers, particularly in the context of smoking tobacco.
The present study had several drawbacks, including the following:
Despite these shortcomings, the study among veterans with HIV adds important knowledge about risk factors for lung cancer.
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