Response in infants Polyclonal activation B Cell types B cell responses to antigens In xid mice (BTK-) Antigen types B Cell Antigen Receptor TI-2TI-1 Mature B cells in general B1 cells, marginal zone B cells YesNo YesNo Yes Bacterial cell wall components Highly repetitive structures Indep.Dep. T cell-independent Follicular B cells No Yes Proteins, CHO, lipids Dep. T cell- dependent

TI-1 antigens - many are bacterial cell wall products, such as LPS, peptidoglycan, lipoprotein, porin - many are recognized by Toll-like receptors - potent but non-specific inducer of B cell proliferation

TI-2 antigens - present on encapsulated bacteria and some viruses - repetitive epitopes with regular spacing - epitopes expressed on a rigid molecular backbone - induce B cell response by cross-linking B cell antigen receptors

TLR CR2 Modulation of TI-2 responses - second signals for activation IL-2, IFN- , IL-3, GMCSF, IL5 T cells, NK cells IC (cognate recognition?) TI-1 antigen (same or diff. Microbe) TACI BLys, APRIL

(1) B1 cells: - Localized to the peritoneal and plueral cavities - Their repertoire is skewed toward reactivity with TI-2 antigens; - the inability of xid mice to respond to TI-2 antigens could be due to the lack of B1 cells in these mice - TI-2 activation of B cells, but not TD or TI-1, in vitro results in a partial induction of the B1 phenotype (2) marginal zone (MZ) B cells - Localized to the perimeter of splenic white pulps - Unlike follicular B cells, MZ B cells are not circulating - with high levels of cell surface complement receptor 2 (CD21) B cells reponsive to TI-2 antigens

TI response to particulate antigen, (PC) Phosphorylcholine + S. pneumoniae MZB1 Spleen Peritoneal cavity Plasmablasts bacteria

Question: How are PC+ MZ B cells activated? Study: Balazs et al, Immunity 17:

MZ B moves to DC But not to M  or neutrophils C56BL/6T-deficient MZ B and DC with bacteria Pre-immune Fig. 1 PC-specific MZ B migrates to DC after immunization

Conclusion of Fig. 1 - MZ B cells in close contact with dendritic cells containing bacteria Experiments further showed: (1) the major cell type that capture bacteria from the blood is CD11c lo dendritic cell (2) CD11c lo dendritic cells move from the blood to the spleen rapidly after immunization, and “present” the bacteria to the antigen-specific MZ B cells (3) soluble factors, Blys and/or APRIL, derived from the CD11c lo dendritic cells support the survival of these antigen-specific B cells; and is important for the full differentiation of the MZ B cells to plasmablasts

Fig. 3 Blood DC (CD11c lo ) or neutrophils that have phagocytosed bacteria migrate to spleen

Fig. 5 - Binding of soluble ligand to TACI is responsible for plasmablast differentiation of PC+ MZ B cells (in vitro)

Fig. 6 Binding of ligand to TACI is responsible for plasmablast differentiation of PC+ MZ B cells (in vivo)

1st division 2nd division CMFDA assay for cell division

Fig. 7 Binding of ligand to TACI is responsible for survival of PC+ MZ B cells

TACI ligands Activation of MZ B cells Plasmablasts Survival & differentiation PC+ MZ B cells TACI PC+ MZ B cells CD11c lo blood DC migrate to spleen after capture of bacteria

Fig.2 In vitro primed BMDC and PEC M  promote plasmablast differentiation from PC+ MZ B cells

Bacteria-primed blood DC promote plasmablast differentiation from PC+ MZ B cells Fig. 4