Infectious Diseases and the Lymphoid Extracellular Matrix Remodeling: A Focus on Conduit System

The conduit system was described in lymphoid organs as a tubular and reticular set of structures compounded by collagen, laminin, perlecan, and heparin sulfate proteoglycan wrapped by reticular fibroblasts. This tubular system is capable of rapidly transport small molecules such as viruses, antigens, chemokines, cytokines, and immunoglobulins through lymphoid organs. This structure plays an important role in guiding the cells to their particular niches, therefore participating in cell cooperation, antigen presentation, and cellular activation. The remodeling of conduits has been described in chronic inflammation and infectious diseases to improve the transport of antigens to specific T and B cells in lymphoid tissue. However, malnutrition and infectious agents may induce extracellular matrix remodeling directly or indirectly, leading to the microarchitecture disorganization of secondary lymphoid organs and their conduit system. In this process, the fibers and cells that compound the conduit system may also be altered, which affects the development of a specific immune response. This review aims to discuss the extracellular matrix remodeling during infectious diseases with an emphasis on the alterations of molecules from the conduit system, which damages the cellular and molecular transit in secondary lymphoid organs compromising the immune response.


Introduction
Extracellular matrix (ECM) comprehends a meshwork of macromolecules such as fibrillar proteins, glycoproteins, enzymes, and proteoglycans, among others. It gives structural and functional support to the cells. ECM is responsible for the development and maintenance of functional activities of organs and allows cellular migration and activation. In secondary lymphoid organs, dendritic cells (DCs), T and B lymphocytes are disposed in a highly organized manner in order to generate an immune response to antigens (Ags). T and B cells are segregated in specific areas, and through the arrangement of ECM, the cells move, interact, and respond to the arrival of Ags. In these organs, in the core of the reticular network, three-dimensional structures of collagen wrapped by fibroblast reticular cells are formed. These 3D structures have been called conduit systems and are in charge of carrying small molecules such as cytokines, chemokines, Ags, and immunoglobulins. Conduits are essential to efficiently transport molecules through the parenchyma to specific regions where they are needed. In this review, the structure and function of ECM and the conduit system of the lymph nodes and spleen are presented, and the consequences of infection-causing remodeling of ECM compounds are addressed. interaction in to establish the adaptative immune response. The ECM plays a fundamental structural and functional role in the distribution of the different cell types, facilitating cell activation, proliferation, migration, and effector function, as described above.
The primary adaptive immune responses initiate at the T cell areas of the secondary lymphoid organs, where naive T cells find DCs, recognize, process, and present antigens on its surface. There was speculation in 1964 about the role of the reticular fibers network of the lymph node in transporting soluble antigens from the inoculation site to the DCs [33]. In the core of the reticular network, a three-dimensional structure is formed. This structure is composed of types I and III collagen wrapped by fibroblast reticular cells (FRC) [22,[34][35][36], known as the conduit system ( Figure 1). Within this specialized structure, soluble low molecular weight molecules, bellow 70 kDa in T cell area [37], are transported from one area to another. It includes cytokines, chemokines, Ag [36], and more recently, it has been shown that even molecules as large as IgM [38]. Recent advances in the structure of the conduit system have demonstrated a pericellular basement membrane surrounding a cell-free lumen composed of specialized matrix compounds. This basement membrane is similar to that underlining endothelial cell, and it contains laminin-411 and laminin-511, which may serve as a barrier [39]. Illustrative scheme of conduit channel. The conduit system is responsible for driving, selectively, proteins, chemokines, and cytokines through the interaction between proteins and proteins associated with collagen fiber into the conduit lumen. Fibrillin molecules maintain the collagen fibers bound between each other and the basal membrane. The conduit lumen is wrapped by an amorphous substance that is secreted by the fibroblast reticular cells. The amorphous Figure 1. Illustrative scheme of conduit channel. The conduit system is responsible for driving, selectively, proteins, chemokines, and cytokines through the interaction between proteins and proteins associated with collagen fiber into the conduit lumen. Fibrillin molecules maintain the collagen fibers bound between each other and the basal membrane. The conduit lumen is wrapped by an amorphous substance that is secreted by the fibroblast reticular cells. The amorphous substance is composed of laminin, heparan sulfate, nidogen, perlecan, fibrillin, and other proteins of basal proteins. The scheme is based on conduit transverse.

Extracellular Matrix and Conduit System Remodeling in Chronic and Infectious Diseases
Secondary lymphoid organs maintain active homeostasis in a steady state. Generally, they are organized as T and B lymphocytes specific areas. These specific compartments are adjacent, allowing cell-to-cell cooperation and activation of the immune response. The adaptive immune response starts at T cell zones. They are composed of a specialized extracellular matrix and the conduit system rich in fibroblast reticular cells. In this area, chemokines and cytokines are expressed by FRC and dendritic cells. They are transported through conduits, which address lymphocytes to specific areas maintaining the organization of compartments in secondary lymphoid organs. During infections, the lymphoid extracellular matrix may suffer dramatic remodeling, which plays a role in the development of a specific immune response. This process improves blood flows, immune cell trafficking, and angiogenesis, resulting in inflammatory reaction and organ enlargement. Thereby, extracellular matrix remodeling promotes the assembly of a pathogen-specific immune response. As an example, lymph nodes are constructed by an intricate network of endothelial and mesenchymal stromal cells. These change their composition after herpes simplex virus type-1 (HSV-1) infection [44]. In this case, the recruitment of lymphocytes to lymph nodes induces the increasing of stromal cell numbers (Lymphoid Stromal Cells -LSC, fibroblast reticular cells, lymphatic endothelial cells, and blood endothelial cells) [44]. The number of fibroblast reticular cells in the inflamed lymph nodes increases as a response to infection, persisting for more than three months to return to the steady-state after pathogen clearance [44]. Most of the proliferation and gene regulation of LSCs occur in the first seven days after infection. After that, they contract gradually [44]. The transcriptional changes result in cell division, antigen presentation, extracellular matrix, apoptosis, and immune response. The observed changes appear to be induced by IFN-α signaling [44]. Because of the activation and growth of FRC, T and B lymphocytes also increase with the lymph node enlargement, and B cell zones remained enlarged for 30 days after infection [44,45]. FRC and lymphatic endothelial cells up-regulate interleukin-7 (IL-7) expression responding to viral infection contributing to lymphocyte survival, remodeling, and reconstruction of the distinct lymph node microenvironment [46].
The extracellular matrix is an active participant in the development of immune response upon infection. Type VII collagen, a compound of extracellular matrix in the skin, the conduits in lymph nodes, and spleen, may capture cochlin from the lumen of the conduit systems [47]. Cochlin is an ECM protein produced by follicular dendritic cells in B cell follicles conduits and plays a role as an innate immune activator [48]. During infectious diseases, cochlin is processed by aggrecanase, releasing LCCL domain that activates macrophages and neutrophils [47]. The importance of the collagen VII-cochlin axis during bacterial infections was evidenced by the reduced IL-6 and IL-1β expression and by the increased bacterial colonization when collagen VII is genetically lost [47]. It had also been demonstrated by the reduced survival of cochlin knockout mice infected with Pseudomonas aeruginosa and Staphylococcus aureus [48].
After pathogen clearance, the inflammatory reaction reduces, and lymphoid organs return to normal size and steady-state. In chronic infections, the persistence of pathogens and antigenic stimulus lead to permanent remodeling that triggers tissue damage. Gradually, the inflammatory reaction is hereby replaced by fibrosis, and in some situations, the changes in the extracellular matrix composition may be irreversible or may alter the function of the organs. This effect has been described in a variety of chronic and infectious diseases. In HIV, the changes in the lymphoid tissue microenvironment are accompanied by fat or fibrosis deposition. They may also be attributed to a loss of leukocytes' communication and the surrounding stromal cells [49]. These cells produce the extracellular matrix components and the growth factors necessary for cell migration, cell proliferation, and lymphoid tissue function [49]. In canine leishmaniasis, the persistence of Leishmania amastigotes induces a chronic inflammatory reaction that ends in a spleen and lymph node fibrosis/collagen deposition [50,51]. Laminin and metallopeptidase-9 are also increased in the spleen of dogs with an advanced infection, suggesting an intense process of extracellular matrix remodeling [50]. In the canine leishmaniasis model, these alterations in the splenic extracellular matrix have been associated to a reduced CXCL13 expression, reduced fibroblast reticular cells (Figure 2), CD4 cells, lymphatic periarteriolar sheath atrophy, lymphoid follicle atrophy, and germinal center disruption (Figure 3) [50][51][52][53].
Some pathogens have developed the ability to use the mechanisms involved in extracellular matrix remodeling to persist and to disseminate inside the host. For example, HIV interacts with fibronectin, one of the components of the extracellular matrix, which facilitates CD4 T lymphocytes infection in vivo [54]. The binding of gp120 envelope protein mediates this interaction with the extracellular matrix to the III1-C region of fibronectin [54]. In the spleen of a chicken model, genotype VI Newcastle disease virus promotes metalloproteinase (MMP)-13 and -14 upregulation and consequent extracellular matrix degradation through collagen breakdown [55]. The authors suggested that, as the extracellular Cells 2020, 9, 725 6 of 13 matrix components interfere with viral spread, extracellular matrix degradation facilitates viral spread, resulting in higher viral load [55]. Collagen destruction was also demonstrated in the spleen of chicken infected with infectious bursal disease virus [56]. Collagen degradation begins three days post-infection in the antigen-trapping zone and impairs tissue organization contributing to permanent immunosuppression [56].
The conduit system also changes during infectious diseases. The fibroblast reticular cell, the main cell that covers the conduit system may be a target during infectious diseases. For instance, in HIV and Simian Immunodeficiency Virus (SIV) infection, the lymph node FRC network is replaced by fibrosis (collagen deposition), impairing the production of IL-7, leading to T cell depletion and immunosuppression [57]. In the mouse model of persistent infection by lymphocytic choriomeningitis virus (LCMV), the FRC network was infected and altered by the action of CD8 T cells [58]. Interestingly, Programmed death -ligand 1 (PD-L1) was up-regulated on FRC, reducing the activation of CD8 T cells and, consequently, the immunopathogenesis, thereby contributing to viral persistence [58].
After pathogen clearance, the inflammatory reaction reduces, and lymphoid organs return to normal size and steady-state. In chronic infections, the persistence of pathogens and antigenic stimulus lead to permanent remodeling that triggers tissue damage. Gradually, the inflammatory reaction is hereby replaced by fibrosis, and in some situations, the changes in the extracellular matrix composition may be irreversible or may alter the function of the organs. This effect has been described in a variety of chronic and infectious diseases. In HIV, the changes in the lymphoid tissue microenvironment are accompanied by fat or fibrosis deposition. They may also be attributed to a loss of leukocytes' communication and the surrounding stromal cells [49]. These cells produce the extracellular matrix components and the growth factors necessary for cell migration, cell proliferation, and lymphoid tissue function [49]. In canine leishmaniasis, the persistence of Leishmania amastigotes induces a chronic inflammatory reaction that ends in a spleen and lymph node fibrosis/collagen deposition [50,51]. Laminin and metallopeptidase-9 are also increased in the spleen of dogs with an advanced infection, suggesting an intense process of extracellular matrix remodeling [50]. In the canine leishmaniasis model, these alterations in the splenic extracellular matrix have been associated to a reduced CXCL13 expression, reduced fibroblast reticular cells (Figure 2), CD4 cells, lymphatic periarteriolar sheath atrophy, lymphoid follicle atrophy, and germinal center disruption (Figure 3) [50][51][52][53]. Depending on the model of study and, consequently, the course of infection, the effects on the FRC may vary. For example, the experimental infection of the murine model with Leishmania infantum leads to an increase in FRC [59]. Upon infection, polynutrient-deficient mice showed a reduction in FRC, dendritic cells, and macrophages when compared with well-nourished mice [59]. Leishmania was co-localized with dendritic cells and high endothelial venules associated with an intact conduit network [59]. In infected and malnourished mice, the authors observed an early parasite visceralization when compared to well-nourished and infected mice. They suggested that early visceralization of amastigotes was not due to a passive movement through a leaking barrier, but to a reduced number of lymph node phagocytes [59]. They further suggested a role of conduit system flow in the early visceralization of Leishmania donovani [59].
Lymphotoxin beta (Ltb) plays a role in splenic architecture, developing conduits along the marginal zone and recruiting CD169+ macrophages [60,61]. In the model of LCMV infection, extracellular distribution of virus along the splenic conduits is necessary for inducing systemic levels of IFN-I and is dependent on the presence of Lymphotoxin B-induced conduits [62]. In the presence of IFN-I, cellular exhaustion is induced through PD-L1 and IL-10 expression inhibiting the response of virus-specific CD8+ T cells and favoring virus persistence [63,64].   Recently, Reynoso et al. [65] demonstrated that vaccinia virus and zika virus are transported through conduits in lymph nodes to have access to cells in order to infect them. They evidenced these viruses rapidly infecting the cells, which were adjacent to conduits [65]. Prions also may traffic through lymph node conduits [66]. Moreover, in a study of the role of perinodal adipose tissue (PAT) during immune responses, the authors observed the fluid of PAT enter the lymph node through PAT-LN conduits contributing to the immune response [67]. Besides, Staphylococcus aureus intradermally or intravenously infected may use PAT-LN conduits to infect PAT [67].
A notable example of lymphoid extracellular matrix remodeling is the formation of tertiary lymphoid tissue during chronic inflammation in various non-lymphoid organs. These structures are able to maintain a cellular organization similar to B and T cell areas of secondary lymphoid organs such as lymph nodes [68]. The maintenance of tertiary lymphoid tissue is dependent on lymphtoxin β that can be expressed by B lymphocytes [68,69]. Tertiary lymphoid tissue is composed of a variety of hematopoietic cells, high endothelial venules, and follicular dendritic cells [69]. An intricate network formed by FRC is also observed, and these cells play an important role in the induction and persistence of tertiary lymphoid tissue since they produce CCL21, express lymphtoxin β receptor and form a functional conduit system [69]. As observed for lymph nodes, conduits are dense in T cell areas and sparser in B cell areas from tertiary lymphoid tissue [68].

Final Considerations
The remodeling process of the extracellular matrix of secondary lymphoid organs plays a vital role during immune responses against infectious agents and has been studied in a variety of models. However, little has been described about the impact of infections on the conduit system. Despite scarce data observed and discussed in this paper, we can conclude that the processes of infection by different etiological agents generate changes in the cellular and fiber components of conduits. In some situations, parasites may use the conduit system or induce changes in it to favor the spread of pathogens and their permanence in the host. During acute through chronic infection, several events aiming to control the microbial spreading result in extracellular matrix remodeling and/or conduit system disruption. For example, after microbial infection the increase of cytokines such as TNF-α and TGF-β together with a high production of matrix metallopeptidases and microbicidal molecules, such as RNS, ROS, and lysosomal enzymes, lead to the tissue damage, breaking or accumulating matrix molecules ( Figure 4). Thus, to avoid such events, it is crucial to maintain an appropriate cooperation and activation of the immune system in secondary lymphoid organs. One question that arises is concerning if killing the microorganism using drugs will further restore the architecture of the organ. Since chronic infection causes intense disorganization and fibrosis, it seems that preventing or limiting such events may be the best way to restore homeostasis. The application of coadjuvant drugs, such as pentoxyfylline [70] and infliximab [71], may protect the tissue from a high production of TNF-α, which is also responsible for the activation of several enzymes, limiting the matrix disorganization. Unfortunately, several other molecules contribute to tissue damage, and further research is needed.