General characteristics of memory cells
Naive lymphocyte differentiation follows two pathways: effector cells are formed (for the ongoing fight against the pathogen) and memory cells (for future protection). A key feature of memory cells is that they do not participate in the primary immune response during which they were formed. Their task is to patrol the body for years (sometimes a lifetime) and await a repeat visit from a familiar antigen.
Evolutionarily, the formation of such a pool of cells is the main advantage of acquired immunity over innate immunity. The high survival rate of these cells is ensured by the active expression of anti-apoptotic molecules Bcl-2 and Bcl-X_L.
Memory B cells
These cells develop in parallel with plasmacytes in the apical zone of germinal centers. Since they originate from mature B centroctyes, critical genetic processes are already complete in them: somatic hypermutation, isotype switching, and affinity maturation.
Features of memory B cells:
- Surface receptors: carry high-affinity isotypes IgG, IgA, or (less commonly) IgE, but completely lack IgD.
- Difference from plasmacytes: immunoglobulins are localized on the membrane (no antibody secretion), and the specific plasmacyte marker CD138 is also absent.
- Molecular profile: formed upon downregulation of the Bcl-6 factor and the mandatory absence of Blimp-1 (which, conversely, is essential for plasma cells).
To migrate to the bone marrow and undergo recirculation, memory B cells downregulate the CXCR5 receptor (which retains them in follicles) and upregulate CXCR4, which responds to the stromal chemokine CXCL12.
Memory T cells: types and homeostasis
They develop from activated CD4+ and CD8+ lymphocytes. An important differentiation marker is the change in RNA splicing of the CD45 molecule: the long CD45RA isoform, which creates steric hindrance, loses some domains and transforms into the shortened CD45R0 version, facilitating future cell activation.
Based on migration properties, memory T cells are divided into:
- Central: retain homing molecules of naive cells (CCR7, CD62L) and continue to circulate through the T-zones of secondary lymphoid organs.
- Effector: lose the CCR7 receptor and cease entering lymph nodes. They acquire integrins and specific chemokine receptors (e.g., CCR9 for the gut, CCR4/CCR10 for the skin), migrating directly to barrier tissues and sites of inflammation.
Interestingly, antigen is not required to maintain the life of memory T cells. Homeostasis is maintained by the cytokines IL-15 and IL-7. To prevent the memory cell pool from overflowing during new infections, the attrition phenomenon operates: older clones uniformly decrease in number under the influence of interferon-alpha.
Secondary immune response
Thanks to the pool of specific memory cells, the response to pathogen re-exposure proceeds entirely differently than the primary one. It is triggered faster (the lag phase is only 1–3 days instead of 4–7), requires lower doses of antigen, and provides reliable protection.
This is most vividly manifested in the humoral branch:
- The intensity of IgG production exceeds the primary level by 100–1000 times, antibodies possess extremely high affinity for the antigen, and they circulate longer.
- Memory cells do not need to re-undergo isotype switching and differentiation.
- The dynamics of IgM production during the secondary response remains weak, at the primary level (since virtually no IgM-bearing variants exist among memory cells; they are generated only during a specific T-independent response).
Briefly: answers to topical questions
What is the average lifespan of various T- and B-memory cell subpopulations in the human body?
The average lifespan of T and B memory cells is very long and comparable to the lifespan of the human body itself.
- Memory B cells persist in the body for years, sometimes for a lifetime. The dependence of their survival on repeated antigen recognition has not been definitively established.
- Memory T cells live for decades. For instance, memory to viral antigens provided by CD8+ T cells can persist for up to 75 years. Their long-term persistence does not require repeated contact with the antigen.
To remember CD45 markers: CD45RA — "A" stands for Absolutely new (naive cell). CD45R0 — "0" stands for Zeroed domain (shortened molecule of an experienced memory cell, without unnecessary hindrance to function).
Class and exam questions
Do memory cells participate in the primary immune response?
No. Although they form during the primary response (typically by weeks 3–4), they remain in reserve. All the fighting against the current infection is carried out by short-lived effector lymphocytes.
What are "surrogate" memory T cells?
This is a pathological pool of cells that forms not from an infection, but due to massive lymphocyte death (lymphopenia). Naive T cells begin to proliferate chaotically under the influence of IL-7, changing their phenotype to memory cells, which threatens to narrow the useful repertoire and trigger autoimmune reactions.
Why is the change of the CD45 isoform necessary during T-cell maturation?
The long CD45RA molecule in naive cells creates steric hindrance. Transitioning to the shortened CD45R0 isoform eliminates these barriers, allowing the memory cell to form an immunological synapse much faster and more easily upon re-encountering the antigen.
What else to study on this topic
- Clonal structure and antigen-recognizing repertoire of memory T cells
- Specificity of chemokine receptors for tissue homing (CCR6, CXCR4, etc.)
- Cytokine regulation of background and homeostatic proliferation (difference in the action of IL-7 and IL-15)
- Molecular and genetic differences between CD8+ memory T cells and cytotoxic lymphocytes (CTLs)
- Mechanism of the memory cell pool attrition phenomenon
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