CD8+ T cell memory is sustained in mice by hepatic stellate cells

Blood defense system - Proposal for a new concept of an immune system against blood borne pathogens comprising the liver, spleen and bone marrow

Blood-borne pathogen (BBP) infections can rapidly progress to life-threatening sepsis and must therefore be promptly eliminated by the host's immune system. Intravascular macrophages of the liver sinusoid, splenic marginal zone and red pulp and perisinusoidal macrophage protrusions in the bone marrow (BM) directly phagocytose BBPs in the blood as an innate immune response. The liver, spleen and BM thereby work together as the blood defence system (BDS) in response to BBPs by exerting their different immunological roles. The liver removes the vast majority of these invading organisms via innate immunity, but their complete elimination is not possible without the actions of antibodies. Splenic marginal zone B cells promptly produce IgM and IgG antibodies against BBPs. The splenic marginal zone transports antigenic information from the innate to the adaptive immune systems. The white pulp of the spleen functions as adaptive immune tissue and produces specific and high-affinity antibodies with an immune memory against BBPs. The BM works to maintain immune memory by supporting the survival of memory B cells, memory T cells and long-lived plasma cells (LLPCs), all of which have dedicated niches. Furthermore, BM perisinusoidal naïve follicular B cells promptly produce IgM antibodies against BBPs in the BM sinusoid and the IgG memory B cells residing in the BM rapidly transform to plasma cells which produce high-affinity IgG antibodies upon reinfection. This review describes the complete immune defence characteristics of the BDS against BBPs through the collaboration of the liver, spleen and BM with combined different immunological roles.

Bone Marrow: The Central Immune System

Bone marrow is known as the site of hematopoiesis. What is not being described in textbooks of immunology is the fact that bone marrow is not only a generative, but also an antigen-responsive, immune organ. It is also a major storage site for antigen-specific memory B and T cells. That bone marrow is a priming site for T cell responses to blood borne antigens was discovered exactly 20 years ago. This review celebrates this important discovery. The review provides a number of examples of medical relevance of bone marrow as a central immune system, including cancer, microbial infections, autoimmune reactions, and bone marrow transplantation. Bone marrow mesenchymal stem cell-derived stromal cells provide distinct bone marrow niches for stem cells and immune cells. By transmitting anti-inflammatory dampening effects, facilitating wound healing and tissue regeneration mesenchymal stem cells contribute to homeostasis of bone and other tissues. Based on the evidence presented, the review proposes that bone marrow is a multifunctional and protective immune system. In an analogy to the central nervous system, it is suggested that bone marrow be designated as the central immune system.