normativa y propuesta de modelos de densificación en lotes

Peripheral deletion is discussed in depth above and is a mechanism of peripheral self-tolerance employed at several B cell development stages. Importantly, anti-self B cells appear to be sensitive to negative selection via BCR ligation at any stage,

transitional B cell stage and perhaps beyond. BCR signals at the transitional B cell stage induce cell death rather than proliferation. Approximately 60% of transitional B cells are deleted at the T2 to mature B cell differentiative step, indicating a major tolerance checkpoint for autoreactive B cells.

3. Anergy

Anergy, defined as functional unresponsiveness, is an important regulation mechanism for autoreactive B cells in the periphery(145, 148-150). There appear to be multiple mechanisms of anergy and which is employed by a given B cell is likely dependent upon the nature of the antigen and the antigen concentration in the

environment. The HEL system again provides insight into this regulation mechanism as anti-HEL B cells become mature, FO B cells but are anergic in the presence of soluble HEL (sHEL) antigen. These anti-HEL B cells were classified as anergic due to their lack of BCR responsiveness, low sIgM expression and negligible response to LPS(145, 151, 152). Another form of anergy was described in the Ars/A1 (anti-ssDNA cross- reactive antigen) model in which the B cells, despite expressing normal levels of IgM, possess attenuated BCR signals and a reduced half-life. In addition, anti-Sm B cells in the 2-12H/Vκ8 are able to mature, maintain sIgM expression, and signal in response to BCR stimulation yet do not proliferate in response to BCR signals. Moreover, they are instrinsically defective in responses to LPS. The mechanism(s) by which anergy is induced in autoreactive B cells is not entirely clear, but it appears the presence of antigen is required for these processes(145, 148-150, 153). For example, constant antigen engagement has been shown to induce BCR uncoupling and subsequent desensitization,

as the Igα/β signaling components of the BCR are sequestered from the external µ

receptor(154-156). Recently DC-secreted factors, including IL-6, CD40L, and TNF-α, have been shown to repress anti-Sm B cells, providing evidence for the involvement of extrinsic factors in the maintenance of B cell anergy(157, 158).

E. DENDRITIC CELLS

Dendritic cells (DCs) are professional antigen presenting cells (APCs) that provide a crucial link between innate and acquired immunity as they are able to both initiate and dictate immune responses(164). There are many different DC subsets with distinct functions during an immune response. Here I will offer further insight into DC immunobiology with particular emphasis on their involvement in B cell regulation.

1. DC Function

DCs, like B cells, are generated in the BM and migrate into peripheral lymphoid and non-lymphoid tissues upon differentiation(165). These professional APCs are largely responsible for ingestion and antigen presentation of foreign antigens and possess specialized detectors, including TLRs (discussed in depth above), that continually

sample their local environment(76). DCs phagocytose foreign pathogens through the use of phagocytic receptors, including Fc receptors (FcRs), CD14, and others(166-168). The goal of DC microbe ingestion is to process these pathogenic antigens and present microbial peptides on their surface MHC molecules to elicit specific T cell help. As I mentioned above, DCs harnass membrane-bound, intact antigen to aid in B cell

stimulation, however T cell activation requires DC-processing and presentation of a particular peptide, as opposed to whole, soluble antigen.

2. DC Maturation

DC function is highly influenced by their activation or maturation status. Immature DCs, which have not been exposed to foreign antigen, have not upregulated co-stimulatory molecules such as CD80, CD86, and CD40, and thus are unable to

engage T cells and subsequently elicit T cell help(173). However, importantly, immature DCs are very efficient at phagocytosis and thus are the important initiators of an immune response(173), since they will be the first antigen presenting cell (APC) to acquire antigen.

Signals through several receptors can induce DC maturation, including CD40, TLRs, and Fas(95, 173-177). Upregulation of these receptors leads to an increase in co- receptor expression, making mature DCs very efficient T cell activators, unlike

immature DCs. After foreign pathogen signals and/or ingestion, DCs acquire a mature phenotype, defined by co-stimulatory molecule upregulation and pro-inflammatory cytokine secretion, including TNF-α, IL-12 and IL-1β(164). Activated, mature DCs are highly migratory and migrate to lymphoid tissues where they encounter T cells. DCs that already reside in lymphoid organs, such as the spleen, tend to be non-migratory and continually sample the local lymphoid environment to aid in immune response

initiation(178-181).

In addition to maturity status, there are many different DC subsets that are also phenotypically and functionally distinct. As my dissertation focuses primarily on murine splenic DCs, I will briefly introduce the splenic subsets: lymphoid, myeloid, and plasmacytoid DCs. Lymphoid DCs (CD11c+CD11b-CD8α+CD4-) are located primarily in the T cell-rich areas of the spleen, including the PALS, and are involved in activating CD8+ CTLs, initiating Th1 differentiation. Myeloid DCs (CD11c+CD11b+CD8α-CD4+/-) are located in the MZ area and thus are able to sample foreign antigens in the blood as it empties into the spleen. Myeloid DCs are able to present to both CD4+ and CD8+ T cells yet predominantly induce Th2 differentiation. Lastly, plasmacytoid DCs

(CD11c+CD11b-B220+Gr1+/- ) are responsible for extensive type 1 interferon production during anti-viral responses(184, 185). Outside of the spleen, specialized DC subsets also exist that are involved in ingesting pathogens from peripheral tissues, including the blood and skin. It is clear distinct anatomical locations, receptor expression profiles, and presentation capabilities demonstrate the highly specialized and vast amount of DC functions in the murine spleen.