Lymph nodes and GCs Flashcards
Identify the marked regions of this cross section of a lymph node.
https://doi.org/10.1016/j.coi.2020.03.007
What are the different kinds of stromal cells in the lymph nodes?
https://doi.org/10.1016/j.coi.2020.03.007
What is the function of fibroblastic reticular cells (FRCs) in the lymph node during the humoral response?
FRCs of the T cell zone mediate the encounter of rare
antigen specific T cells with antigen-presenting DCs resulting in T cell activation. FRCs produce CCL19/21 (ligands for CCR7), and the movement of both activated APCs and T cells from the periphery is highly dependent on APC and T cell expression of CCR7. CCL19/21 gradients lead APCs and T cells to the T cell zone of lymph nodes.
https://doi.org/10.1016/j.coi.2020.03.007
What is the function of CXCL12-producing reticular cells (CRCs) during the humoral response?
The dark zone is supported by a network of CRCs which generate a CXCL12 gradient facilitating the localization of CXCR4+ GC B cells for proliferation and somatic hypermutation in this compartment.
https://doi.org/10.1016/j.coi.2020.03.007
What is the function of follicular dendritic cells (FDCs) during the humoral response?
The dark zone is supported by a network of CRCs which generate a CXCL12 gradient facilitating the localization of CXCR4+ GC B cells for proliferation and somatic hypermutation in this compartment. GC B cells then upregulate CXCR5 and migrate towards the CXCL13 gradient produced by FDCs in the light zone. Here, GC B cells test their newly mutated receptors on antigen presented on FDCs (FDCs have complement receptors CD35 and CD32 that can capture complement-coated antigen and “hold” it out for B cells to encounter) and process it for presentation to Tfh cells that in turn provide survival signals. Positively selected GC B cells can then migrate back to the dark zone and undergo further rounds of proliferation and somatic hypermutation.
https://doi.org/10.1016/j.coi.2020.03.007
What is the function of marginal reticular cells (MRCs) in the lymph node during the humoral response?
During humoral immune responses, activation of lymphocytes by antigen is the necessary first step for the formation of GCs. FRCs of the T cell zone mediate the encounter of rare antigen specific T cells with antigen-presenting DCs resulting in T cell activation. Antigen can also enter the LN through the subcapsular sinus, which is lined with LECs, SSMs and MRCs. These cells form a specialized niche that promotes the transport of antigen to B cells in the follicle for activation. MRCs specifically do this by playing an essential role in the differentiation of specialized macrophages in the lymph node that then transport antigen to B cells.
https://doi.org/10.1016/j.coi.2020.03.007
Describe the steps in the germinal center reaction (GCR)
For B cells to participate in the GC response, they first need to recognize their cognate antigen via their BCR. B cells are able to directly bind soluble antigen or bind antigen presented on the surface of follicular dendritic cells (FDCs), macrophages or dendritic cells. Once activated by antigen encounter, B cells upregulate the chemokine receptor CCR7, which facilitates the migration of B cells via a chemokine gradient toward the CCR7 ligands CCL19 and CCL21 expressed in the T cell zone. At the interface between the B cell follicle and the T cell zone (T:B border), B cells present fragments of peptide antigen on major histocompatibility complex (MHC)-Class II to CD4+ helper T cells that provide them with survival and co-stimulatory signals. B cells will then divide at the perimeter of the follicle and will either initiate the GC response or differentiate into short-lived extrafollicular plasma cells or memory B cells. Extrafollicular plasma cells produce the first wave of antibodies before undergoing apoptosis within a few days, providing an initial burst of antibodies that are essential for early control of infection while the GC response is established.
After cognate interactions with CD4+ T cells, activated B cells will migrate to the center of the follicle to seed the GC response. These GC B cell precursors begin to rapidly divide and undergo clonal expansion during which the GC is divided into two distinct compartments known as the dark zone (DZ) and the light zone (LZ). The DZ contains the rapidly diving B cells known as centroblasts, which undergo SHM. Centroblasts express the chemokine receptor CXCR4 whose ligand, CXCL12, is produced by stromal cells in the DZ (CXCL12-expressing reticular cells, CRCs). This chemokine localizes the centroblasts within the DZ, thereby generating GC polarity. Once GC B cells have undergone SHM in the DZ, they downregulate CXCR4 and migrate to the LZ, to receive positive selection signals. The LZ is rich in FDCs that produce CXCL13, which attracts GC B cells that exit the DZ as centrocytes, through their expression of CXCR5. The LZ also contains Tfh and Tfr cells that are important for the successful and regulated continuation of the GC response. FDCs and Tfh cells are critical for the positive selection of centrocytes, while Tfr cells are thought to regulate the output of the GC response. Together, these processes culminate in the emergence of long-lived antibody secreting plasma cells and memory B cells whose BCRs bind antigen with high affinity.
https://www.frontiersin.org/articles/10.3389/fimmu.2018.02469/full
https://doi.org/10.4049/jimmunol.1800986
Describe the structure of a lymph node.
https://www.frontiersin.org/articles/10.3389/fimmu.2018.02469/full
Describe the interaction between Tfh cells and antigen-presenting DCs and Tfh and antigen-presenting B cells in the GCR.
Naïve CD4+ T cells interact with DC in the T-cell zones of lymph nodes or in the interfollicular zones. In humans, IL-12 is the most important cytokine produced by DCs capable to activate STAT4 in CD4+ T cells to produce IL-21. This cytokine then feedbacks to the same cells engaging the IL-21R which activates STAT3 which is responsible for the differentiation. These cells upregulate the expression of the transcription factor Ascl2 and BCL-6 to become early T follicular helper (Tfh) cells. These transcription factors upregulate CXCR5 and downregulate CCR7. STAT5 is a negative regulator of Tfh differentiation through upregulation of Blimp-1. Tfh cells migrate to the T:B cell border and differentiate into germinal center Tfh cells. Other cytokines produced by DCs (IL-27 and IL-6) and B cells (IL-6) contribute to the activation of STAT3. IL-21 also activates IL-21R on B cells increasing the expression of BCL-6. GC Tfh cells and certain subsets of peripheral blood Tfh cells can produce IL-4 which acts on IL-4R present on B cells. Cell–cell interactions between activated CD4+ T cells and DCs (ICOS–ICOSL) or between CD4+ T cells and B cells (CD28–CD86, ICOS–ICOSL, PD-1– PD-1L, CD40–CD40L, etc.) also play a fundamental role in Tfh cell functions.
Describe the method by which Tfr cells in the GC suppress the GCR.
Tfr cells regulate T – B interactions within the germinal center (GC) by physical interference at the immunological synapse, which is required for the survival feedback loop between GC B cells and T follicular helper (Tfh) cells. CTLA-4 is a key molecule of T follicular regulatory (Tfr) cell function at immunological synapse, as it directly blocks CD80/CD86 co-stimulatory signals. Using these mechanisms, Tfr cells impair GC B cell metabolism (mainly by decreasing glucose uptake and usage) and induce the downregulation of GC B cell effector molecules, such as Pou2af1 (required for GC B cell formation), Xbp1 (required for antibody secretion), and Aicda (required for class switch recombination). On the other side of the immunological synapse, Tfr cells limit IL-21, and IL-4 secretion by Tfh cells. Granzyme B, IL-10, and TGF-β secretion by Tfr cells may also account for their regulatory capacity.
https://www.frontiersin.org/articles/10.3389/fimmu.2018.02469/full
What is the timeline for the formation of germinal centers after encountering antigen from infection or vaccination?
On day 0, B cells and T cells are activated by recognition of their cognate antigen in the primary follicle and T cell zone, respectively. On day 1, the activated B cells and T cells migrate to the interfollicular region and start to interact. On day 2, B cells and T cells form long-lived interactions, resulting in the full activation of B cells. T cells acquire the characteristic T follicular helper cell (TFH cell) phenotype. On day 3, TFH cells migrate from the interfollicular region into the follicle. Some antigen-activated B cells differentiate into antibody-secreting cells or early plasmablasts that move to a region adjacent to the subcapsular sinus (SCS). On day 4, B cells migrate from the interfollicular region into the centre of the follicle — which is characterized by a network of follicular dendritic cells (FDCs) — begin to proliferate and, as a result, push the resident follicular B cells aside to form the early germinal center (GC), which consists of B cell blasts surrounded by the mantle zone. This structure is also referred to as the secondary follicle, thus distinguishing it from the ‘GC-less’ primary follicle. On days 5–6, the GC rapidly expands as a result of the fast proliferation of the B cell blasts. On day 7, dark zones and light zones form, which results in the establishment of the mature GC.
https://www.nature.com/articles/nri3804
