Chronic Inflammation as a Contributing Cause of B Cell Decline in Aging
B cells are important to the coordination of the immune response. Dysfunctional B cells emerge with age, however, leading to autoimmunity and contributing to immunosenescence, the name given to the general age-related decline in effectiveness of the immune system. Animal studies have shown that selective destruction of the entire B cell population is beneficial in older individuals, improving the immune response: the cells are quickly replaced, but the harmful portion will take much longer to reemerge. Setting all of this to one side, the open access review here is largely focused on more subtle changes in the B cell population and its production in the bone marrow, driven by the effects of age-related chronic inflammation on stem cells and progenitor cells.
The alterations of the B-cell compartment in aging have been evaluated by contrast to B-cell physiology in young adults. Overall, B-cell generation and function demonstrate large similarities between young mice and humans. In the more detailed mouse context, B cells arise from uncommitted progenitors nested in the bone marrow. Overall, aging disturbs B-cell development in the mouse bone marrow. Strikingly, aging seems to introduce a high mouse-to-mouse variability in early progenitor B cellularity compared to young mice. Impaired B-cell development occurs as a result of affected RAG and SLC expression, as well as decreased sensitivity to IL-7 signals. The in-depth situation in humans remains to be established. Nevertheless, available studies suggest that the amount of B cells decreases, although proportions of progenitor and mature B subpopulations may not be substantially changed in the aged bone marrow.
Various clues point at a role for inflammation in the altered B-cell development in aging, albeit the data is generally based on similarities with acute inflammatory responses. Indeed, pro-inflammatory senescent cells as well as terminally differentiated CD8+ effector T cells accumulate in the bones of old mice and humans respectively. Correspondingly, concentrations of pro-inflammatory molecules or their production by cells are increased in bone marrow during aging. The balance between the negating effects of anti-inflammatory cytokines and the intensity, as well as variety, of pro-inflammatory molecules expressed could contribute to the observed variability in the deterioration of early B-cell development in susceptible aged organisms.
Inflammation can affect the differentiation of multi-lineage hematopoietic progenitors. Thus, aged mouse and human hematopoietic stem cell (HSC) physiology is altered and the output of this compartment reveals a bias against the production of lymphocytes due to the accumulation of stem cells with a propensity to differentiate into myeloid cells. This limits the production of B cells. Persistent inflammation in aging could also compromise the differentiation of B lineage-restricted progenitor cells. A major effect of aging is the repression in B cells of the expression and/or activity of the transcription factors E2A and EBF1, which control the RAG and SLC genes.
Despite the alteration of the B-cell compartment in the bone marrow, the cellularity of mature B cells in the spleen is comparable between aged and young mice. However, this apparent stability masks underlying disparities in the distribution of mature B-cell subsets. The situation in humans appears more difficult to appreciate, since most studies performed analyses based on blood samples, which have an inherent variability and may not reflect mature B-cell representation within tissues. Altogether, the B-cell fraction in the blood of elderly people appears decreased and the proportions of naïve and memory subsets altered. Similar to the bone marrow, various inflammatory molecules could influence the distribution of mature B-cell populations.