The Role of Dendritic Cells in S. pneumoniae Transport to Follicular Dendritic Cells

Mechanical control of antigen detection and discrimination by T and B cell receptors

The adaptive immune response is orchestrated by just two cell types, T cells and B cells. Both cells possess the remarkable ability to recognize virtually any antigen through their respective antigen receptors-the T cell receptor (TCR) and B cell receptor (BCR). Despite extensive investigations into the biochemical signaling events triggered by antigen recognition in these cells, our ability to predict or control the outcome of T and B cell activation remains elusive. This challenge is compounded by the sensitivity of T and B cells to the biophysical properties of antigens and the cells presenting them-a phenomenon we are just beginning to understand. Recent insights underscore the central role of mechanical forces in this process, governing the conformation, signaling activity, and spatial organization of TCRs and BCRs within the cell membrane, ultimately eliciting distinct cellular responses. Traditionally, T cells and B cells have been studied independently, with researchers working in parallel to decipher the mechanisms of activation. While these investigations have unveiled many overlaps in how these cell types sense and respond to antigens, notable differences exist. To fully grasp their biology and harness it for therapeutic purposes, these distinctions must be considered. This review compares and contrasts the TCR and BCR, placing emphasis on the role of mechanical force in regulating the activity of both receptors to shape cellular and humoral adaptive immune responses.

Sepsis-Related Lung Injury and the Complication of Extrapulmonary Pneumococcal Pneumonia

Globally, sepsis and pneumonia account for significant mortality and morbidity. A complex interplay of immune-molecular pathways underlies both sepsis and pneumonia, resulting in similar and overlapping disease characteristics. Sepsis could result from unmanaged pneumonia. Similarly, sepsis patients have pneumonia as a common complication in the intensive care unit. A significant percentage of pneumonia is misdiagnosed as septic shock. Therefore, our knowledge of the clinical relationship between pneumonia and sepsis is imperative to the proper management of these syndromes. Regarding pathogenesis and etiology, pneumococcus is one of the leading pathogens implicated in both pneumonia and sepsis syndromes. Growing evidence suggests that pneumococcal pneumonia can potentially disseminate and consequently induce systemic inflammation and severe sepsis. Streptococcus pneumoniae could potentially exploit the function of dendritic cells (DCs) to facilitate bacterial dissemination. This highlights the importance of pathogen-immune cell crosstalk in the pathophysiology of sepsis and pneumonia. The role of DCs in pneumococcal infections and sepsis is not well understood. Therefore, studying the immunologic crosstalk between pneumococcus and host immune mediators is crucial to elucidating the pathophysiology of pneumonia-induced lung injury and sepsis. This knowledge would help mitigate clinical diagnosis and management challenges.

The follicular dendritic cell: At the germinal center of autoimmunity?

Autoimmune diseases strain healthcare systems worldwide as their incidence rises, and current treatments put patients at risk for infections. An increased understanding of autoimmune diseases is required to develop targeted therapies that do not impair normal immune function. Many autoimmune diseases present with autoantibodies, which drive local or systemic inflammation. This indicates the presence of autoreactive B cells that have escaped tolerance. An important step in the development of autoreactive B cells is the germinal center (GC) reaction, where they undergo affinity maturation toward cognate self-antigen. Follicular dendritic cells (FDCs) perform the essential task of antigen presentation to B cells during the affinity maturation process. However, in recent years, it has become clear that FDCs play a much more active role in regulation of GC processes. Here, we evaluate the biology of FDCs in the context of autoimmune disease, with the goal of informing future therapeutic strategies.

Salmonella infection induces the reorganization of follicular dendritic cell networks concomitant with the failure to generate germinal centers

Germinal centers (GCs) are sites where plasma and memory B cells form to generate high-affinity, Ig class-switched antibodies. Specialized stromal cells called follicular dendritic cells (FDCs) are essential for GC formation. During systemic Salmonella Typhimurium (STm) infection GCs are absent, whereas extensive extrafollicular and switched antibody responses are maintained. The mechanisms that underpin the absence of GC formation are incompletely understood. Here, we demonstrate that STm induces a reversible disruption of niches within the splenic microenvironment, including the T and B cell compartments and the marginal zone. Alongside these effects after infection, mature FDC networks are strikingly absent, whereas immature FDC precursors, including marginal sinus pre-FDCs (MadCAM-1+) and perivascular pre-FDCs (PDGFRβ+) are enriched. As normal FDC networks re-establish, extensive GCs become detectable throughout the spleen. Therefore, the reorganization of FDC networks and the loss of GC responses are key, parallel features of systemic STm infections.

Streptococcus pneumoniae interactions with the complement system

Host innate and adaptive immunity to infection with Streptococcus pneumoniae is critically dependent on the complement system, demonstrated by the high incidence of invasive S. pneumoniae infection in people with inherited deficiency of complement components. The complement system is activated by S. pneumoniae through multiple mechanisms. The classical complement pathway is activated by recognition of S. pneumoniae by C-reactive protein, serum amyloid P, C1q, SIGN-R1, or natural or acquired antibody. Some S. pneumoniae strains are also recognised by ficolins to activate the mannose binding lectin (MBL) activation pathway. Complement activation is then amplified by the alternative complement pathway, which can also be activated by S. pneumoniae directly. Complement activation results in covalent linkage of the opsonic complement factors C3b and iC3b to the S. pneumoniae surface which promote phagocytic clearance, along with complement-mediated immune adherence to erythrocytes, thereby protecting against septicaemia. The role of complement for mucosal immunity to S. pneumoniae is less clear. Given the major role of complement in controlling infection with S. pneumoniae, it is perhaps unsurprising that S. pneumoniae has evolved multiple mechanisms of complement evasion, including the capsule, multiple surface proteins, and the toxin pneumolysin. There is considerable variation between S. pneumoniae capsular serotypes and genotypes with regards to sensitivity to complement which correlates with ability to cause invasive infections. However, at present we only have a limited understanding of the main mechanisms causing variations in complement sensitivity between S. pneumoniae strains and to non-pathogenic streptococci.

Pathogen Receptor Membrane-Coating Facet Structures Boost Nanomaterial Immune Escape and Antibacterial Performance

Nanomaterials show great potential for the treatment of bacterial infections, but their effects remain limited by low antibacterial efficiency and immune clearance. Facet-dependent nanozymes coated with pathogen receptor membranes were fabricated, providing an approach for producing superphotothermal antibacterial nanomaterials with high biocompatibility and low immune clearance. (100)- and (112)-Faceted CuFeSe₂ presented excellent photothermal conversion efficiency (46%). However, the peroxidase-like activity of (100)-faceted CuFeSe₂ enhanced the decomposition of H₂O₂ to hydroxyl radicals (^•OH) and was markedly greater than that of (112)-faceted CuFeSe₂, with nearly 100% of Staphylococcus aureus being killed under near-infrared (NIR) irradiation. Importantly, bacteria-pretreated immune membranes (i.e., pathogen receptor membranes) coated with CuFeSe₂ exhibited superior S. aureus-binding ability, presented obvious immune-evading capability, and resulted in targeted delivery to infected sites. As a proof-of-principle demonstration, these findings hold promise for the use of pathogen receptor membrane-coated facet-dependent nanomaterials in clinical applications and the treatment of bacterial infections.

Characterization of human FDCs reveals regulation of T cells and antigen presentation to B cells

Stromal-derived follicular dendritic cells (FDCs) are essential for germinal centers (GCs), the site where B cells maturate their antibodies. FDCs present native antigen to B cells and maintain a CXCL13 gradient to form the B cell follicle. Yet despite their essential role, the transcriptome of human FDCs remains undefined. Using single-cell RNA sequencing and microarray, we provided the transcriptome of these enigmatic cells as a comprehensive resource. Key genes were validated by flow cytometry and microscopy. Surprisingly, marginal reticular cells (MRCs) rather than FDCs expressed B cell activating factor (BAFF). Furthermore, we found that human FDCs expressed TLR4 and can alter antigen availability in response to pathogen-associated molecular patterns (PAMPs). High expression of PD-L1 and PD-L2 on FDCs activated PD1 on T cells. In addition, we found expression of genes related to T cell regulation, such as HLA-DRA, CD40, and others. These data suggest intimate contact between human FDCs and T cells.

External cues to drive B cell function towards immunotherapy

Immunotherapy stands out as a powerful and promising therapeutic strategy in the treatment of cancer, infections, and autoimmune diseases. Adoptive immune therapies are usually centered on modified T cells and their specific expansion towards antigen-specific T cells against cancer and other diseases. However, despite their unmatched features, the potential of B cells in immunotherapy is just beginning to be explored. The main role of B cells in the immune response is to secrete antigen-specific antibodies and provide long-term protection against foreign pathogens. They further function as antigen-presenting cells (APCs) and secrete pro- and anti-inflammatory cytokines and thus exert positive and negative regulatory stimuli on other cells involved in the immune response such as T cells. Therefore, while hyperactivation of B cells can cause autoimmunity, their dysfunctions lead to severe immunodeficiencies. Only suitably activated B cells can play an active role in the treatment of cancers, infections, and autoimmune diseases. As a result, studies have focused on B cell-targeted immunotherapies in recent years. For this, the development, functions, interactions with the microenvironment, and clinical importance of B cells should be well understood. In this review, we summarize the main events during B cell activation. From the viewpoint of mechanobiology we discuss the translation of external cues such as surface topology, substrate stiffness, and biochemical signaling into B cell functions. We further dive into current B cell-targeted therapy strategies and their clinical applications. STATEMENT OF SIGNIFICANCE: B cells are proving as a promising tool in the field of immunotherapy. B cells exhibit various functions such as antibody production, antigen presentation or secretion of immune-regulatory factors which can be utilized in the fight against oncological or immunological disorders. In this review we discuss the importance of external mechanobiological cues such as surface topology, substrate stiffness, and biochemical signaling on B cell function. We further summarize B cell-targeted therapy strategies and their clinical applications, as in the context of anti-tumor responses and autoimmune diseases.

Memory B Cells in Pregnancy Sensitization

Memory B cells play an important role in immunity to pathogens as these cells are poised to rapidly differentiate into antibody-secreting cells upon antigen re-encounter. Memory B cells also develop over the course of HLA-sensitization during pregnancy and transplantation. In this review, we discuss the potential contribution of memory B cells to pregnancy sensitization as well as the impact of these cells on transplant candidacy and outcomes. We start by summarizing how B cell subsets are altered in pregnancy and discuss what is known about HLA-specific B cell responses given our current understanding of fetal antigen availability in maternal secondary lymphoid tissues. We then review the molecular mechanisms governing the generation and maintenance of memory B cells during infection - including the role of T follicular helper cells - and discuss the experimental evidence for the development of these cells during pregnancy. Finally, we discuss how memory B cells impact access to transplantation and transplant outcomes for a range of transplant recipients.

Understanding the Functional Properties of Neonatal Dendritic Cells: A Doorway to Enhance Vaccine Effectiveness?

Increased susceptibility to infectious diseases is a hallmark of the neonatal period of life that is generally attributed to a relative immaturity of the immune system. Dendritic cells (DCs) are innate immune sentinels with vital roles in the initiation and orchestration of immune responses, thus, constituting a promising target for promoting neonatal immunity. However, as is the case for other immune cells, neonatal DCs have been suggested to be functionally immature compared to their adult counterparts. Here we review some of the unique aspects of neonatal DCs that shape immune responses in early life and speculate whether the functional properties of neonatal DCs could be exploited or manipulated to promote more effective vaccination in early life.

A model for the regulation of follicular dendritic cells predicts invariant reciprocal-time decay of post-vaccine antibody response

Follicular dendritic cells (FDC) play a crucial role in the regulation of humoral immunity. They are believed to be responsible for long-term persistence of antibody, due to their role in antibody response induction and their ability to retain antigen in immunogenic form for long periods. In this article, a regulatory control model is proposed which links persistence of humoral immunity with cellular processes associated with FDCs. The argument comprises three elements. The first is a literature review of population-level studies of post-vaccination antibody persistence. It is found that reciprocal-time (∝1/t) decay of antibody levels is widely reported, over a range of ages, observation times and vaccine types. The second element is a mathematical control model for cell population decay for which reciprocal-time decay is a stable attractor. Additionally, control effectors are easily identified, leading to models of homeostatic control of the reciprocal-time decay rate. The final element is a literature review of FDC functionality. This reveals a striking concordance between cell properties required by the model and those widely observed of FDCs, some of which are unique to this cell type. The proposed model is able to unify a wide range of disparate observations of FDC function under one regulatory principle, and to characterize precisely forms of FDC regulation and dysregulation. Many infectious and immunological diseases are increasingly being linked to FDC regulation, therefore a precise understanding of the underlying mechanisms would be of significant benefit for the development of new therapies.

Epithelial-mesenchymal transition in cancer metastasis through the lymphatic system

It was already in the 18th century when the French surgeon LeDran first noted that breast cancer patients with spread of tumor cells to their axillary lymph nodes had a drastically worse prognosis than patients without spread (LeDran et al., ). Since then, metastatic spread of cancer cells to regional lymph nodes has been established as the most important prognostic factor in many types of cancer (Carter et al., ; Elston and Ellis, ). However, despite its clinical importance, lymph metastasis remains an underexplored area of tumor biology. Fundamental questions, such as when, how, and perhaps most importantly, why tumor cells disseminate through the lymphatic system, remain largely unanswered. Accordingly, no treatment strategies exist that specifically target lymph metastasis. The identification of epithelial-mesenchymal transition (EMT) as a mechanism, which allows cancer cells to dedifferentiate and acquire enhanced migratory and invasive properties, has been a game changer in cancer research. Conceptually, EMT provides an explanation for why epithelial cancers with poor differentiation status are generally more aggressive and prone to metastasize than more differentiated cancers. Inflammatory cytokines, such as TGF-β, which are produced and secreted by tumor-infiltrating immune cells, are potent inducers of EMT. Thus, reactivation of EMT also links cancer-related inflammation to invasive and metastatic disease. Recently, we found that breast cancer cells undergoing TGF-β-induced EMT acquire properties of immune cells allowing them to disseminate in a targeted fashion through the lymphatic system similar to activated dendritic cells during inflammation. Here, we review our current understanding of the mechanisms by which cancer cells spread through the lymphatic system and the links to inflammation and the immune system. We also emphasize how imaging techniques have the potential to further expand our knowledge of the mechanisms of lymph metastasis, and how lymph nodes serve as an interface between cancer and the immune system.