One of the distinct characteristics of Adaptive Immunity is Immunological Memory. Immunological memory aids the immune system to mount a faster and more efficient response to reinfection. However, this exclusivity has been challenged in the last two decades with evidence coming forth of immunological memory exhibited in innate immunity. The memory characteristic of innate immunity is termed as 'Trained Immunity' and it results in an elevated response to secondary infection.
It is important to note that despite being the same concept, the functioning of the memory characteristic of both types of immunity varies greatly. Unlike adaptive immunity which utilizes genetic recombination, trained immunity is mediated by epigenetic reprogramming of the transcriptional pathways within cells.
Trained immunity is said to last from 3 months to 1 year and in cases of live vaccines up to 5 years as opposed to adaptive immunological memory that lasts for an individual's lifetime.
Another interesting feature of trained immunity is its non-specificity. After an infection, the innate immune system is 'primed' for a stronger response to reinfection not only to the primary pathogen but also to several other unrelated ones.
An example of the non-specific nature of trained immunity is the protection provided by the BCG vaccine. Apart from priming the immune system against Mycobacterium tuberculosis, it also provides protection against Candida albicans and Schistosoma mansoni. The response to these unrelated pathogens has been confirmed to be produced by the innate immune system.
The BCG vaccine can also be applicable in cases of cancer treatments since studies have shown successful tumour remission as a result of trained immunity. The BCG vaccine is currently being employed as an FDA approved immunotherapeutic strategy to treat bladder cancer in the USA and is also used in some cases of lymphomas and melanomas.
Trained Immunity is a new discovery that drastically changes our view of innate immunity and the immune system as a whole. These new findings might lend us hand in understanding the several unexplained and unanswered questions that we still have about the system that has evolved with the sole aim to protect us. Trained immunity could also explain the evolutionary success of 97% of earth's lifeforms that depend upon innate immunity for protection against pathogens since adaptive immunity is an exclusive feature of vertebrates. Some studies suggest a link between trained immunity and autoimmune diseases. More knowledge about trained immunity could also usher in a new era of vaccines that target the innate immune system. Trained Immunity is a new and exciting avenue for future research in Immunology.
Netea, M. G., et al, (2020). Defining trained immunity and its role in health and disease. Nature Reviews Immunology, 20(6), 375–388. https://doi.org/10.1038/s41577-020-0285-6
