Measles – A Killer of Immune Memory

Measles was once a very common childhood disease that was rampant worldwide. Measles is caused by the Rubeola virus, is spread via respiratory droplets in coughs and sneezes, and is considered one of the most contagious viruses. Disease begins with high fever, red, watery eyes, coughing, and sore throat, followed a few days later by a reddish, raised rash. Typically the rash lasted around a week before most patients recovered fully. However, one of the long known clinical observations was that patients with measles tended to be very susceptible to other infections during and after measles, particularly bacterial infections. In the pre-vaccine era, this measles effect was linked to a high percentage of childhood deaths from other infections. While this increased susceptibility was well documented, a basis for the effect was never adequately determined.

In recent years, lack of vaccination compliance has led to an alarming resurgence of measles cases, with 2019 already seeing the most cases in the United States since 1992. Not only are these individuals suffering from measles itself, but all are likely going to endure an increased risk for other infections during the months to years after recovery from measles. Two recent studies now shed new light on how measles accomplishes this dramatic aftereffect. Both studies reveal a nefarious attack by measles on our B cells. Whenever we encounter foreign substances our B cells are primed to produce antibodies, a process that continues throughout our lives. When we encounter a pathogen for the first time, the antibodies that are produced are critical for eliminating the pathogen and allowing us to recover. Still, this initial antibody production by B cells takes roughly a week, so we may become ill before sufficient antibody is produced to squelch the infection. After the infection resides most B cells die off, but a special subset called memory B cells persists. Memory B cells are very long-lived and continually surveil our bodies for their target pathogen. Every future time that we encounter the target pathogen the memory B cells very quickly produce antibodies and prevent infection. This is also how most vaccines work, by inducing memory B cells whose antibodies will protect us from any future infection from the vaccine target. So your memory B cell population represents your collective immune memory to every foreign substance and pathogen you’ve encountered in your life.

In the first of the two recent studies, researchers found that measles infection leads to a drastic reduction in the memory B cells population. By depleting these memory B cells, measles is essentially wiping out much of the immunity that has been accumulated since birth. Because of this “immune amnesia”, the individual is no longer protected against many previously encountered pathogens so is much more susceptible to diseases. In the companion study, measles infection was shown to significantly reduce the antibody repertoire two months after infection, again putting these children at higher risk for secondary infection. Both of these studies were short-term, but previous studies have indicated that the measles effect on immunity may persist for as long as 5 years. None of these deleterious effects were seen with measles vaccinated children, so vaccination protects not only from acute measles, but also from the lingering immune impairment that can lead to other serious infections.

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