254 PART II Generation of B-Cell and T-Cell Responses rearrangement of their endogenous light-chain DNA. Some TI-2 antigens activate B cells by extensively crosslinking of these cells succeed in replacing the k light chain of the self- the mlg receptor. However, TI-2 antigens differ from TI-1 antigen reactive antibody with a x chain encoded by endoge- antigens in three important respects. First, they are not B-cell nous A-chain gene segments. As a result, these cells will begin mitogens and so do not act as polyclonal activators. Second to express an"edited"mlg M with a different light chain and a TI-l antigens will activate both mature and immature B cells, specificity that is not self-reactive. These cells escape negative but TI-2 antigens activate mature B cells and inactivate im selection and leave the bone marrow mature B cells. Third, although the B-cell response to TI-2 antigens does not require direct involvement of TH cells cytokines derived from TH cells are required for efficient B-Cell Activation and Proliferation B-cell proliferation and for class switching to isotypes other than IgM. This can be shown by comparing the effect of TI-2 After export of B cells from the bone marrow, activation, pro- antigens in mice made T-cell-deficient in various ways. In liferation, and differentiation occur in the periphery and re- nude mice, which lack thymus-derived Tcells but do contain quire antigen Antigen-driven activation and clonal selection a few t cells that arise from other sites that probably lie in the of naive B cells leads to generation of plasma cells and mem- intestine, TI-2 antigens do elicit B-cell responses. TI-2 anti- ory B cells. In the absence of antigen-induced activation, gens do not induce antibody production in mice that cannot naive B cells in the periphery have a short life span, dying express either aB or y8 TCRs because the genes encoding the within a few weeks by apoptosis(see Figure 11-1) TCR B and 8 chains have been knocked out. Administration of a few t cells to these tcr-knockout mice restores their Thymus-Dependent and Thymus Independent Antigen Have Different ability to elicit B山 esponses to ncin dependent antigens is Requirements for Response different from the response to thymus-dependent antigens Table 11-2). The response to Ti antigens is generally weaker, Depending on the nature of the antigen, B-cell activation pro- no memory cells are formed, and igM is the predominant ceeds by two different routes, one dependent upon TH cells, the antibody secreted, reflecting a low level of class switching other not. The B-cell response to thymus-dependent (TD)an- These differences highlight the important role played by th tigens requires direct contact with TH cells, not simply expo- cells in generating memory B cells, affinity maturation, and sure to TH-derived cytokines. Antigens that can activate B cells class switching to other isotypes in the absence of this kind of direct participation by TH cells are known as thymus-independent (TD) antigens. TI antigens are Two Types of Signals Drive B Cells into divided into types 1 and 2, and they activate B cells by different and Through the Cell Cycle mechanisms. Some bacterial cell-wall components, including lipopolysaccharide(LpS), function as type 1 thymus-independent Naive, or resting, B cells are nondividing cells in the go stage of (TI-1)antigens. Type 2 thymus-independent (T1-2) antigens are the cell cycle. Activation drives the resting cell into the cell cy highly repetitious molecules such as polymeric proteins(e.g, cle, progressing through Gn into the S phase, in which dNA is bacterial flagellin or bacterial cell-wall polysaccharides with replicated. The transition from g1 to S is a critical restriction repeating polysaccharide units. point in the cell cycle. Once a cell has reached S, it completes Most TI-1 antigens are polyclonal B-cell activators(mito- the cell cycle, moving through Ga and into mitosis(M) gens); that is, they are able to activate b cells regardless of Analysis of the progression of lymphocytes from go to the their antigenic specificity. At high concentrations, some TI-1 S phase revealed similarities with the parallel sequence in fi- antigens will stimulate proliferation and antibody secretion broblast cells. These events could be grouped into two cate- which TI-1 antigens activate B cells is not well understood. tence signals drive the b cell from Go into early GI, rendering When B cells are exposed to lower concentrations of TI-l the cell competent to receive the next level of signals. Pro- antigens, only those B cells specific for epitopes of the antigen gression signals then drive the cell from gi into S and ulti will be activated. These antigens can stimulate antibody pro- mately to cell division and differentiation. Competence is duction in nude mice(which lack a thymus and thus are achieved by not one but two distinct signaling events, which greatly deficient in T cells), and the response is not greatly are designated signal 1 and signal 2. These signaling events are augmented by transferring T cells into these athymic mice, generated by different pathways with thymus-independent indicating that Ti-1 antigens are truly T-cell independent. and thymus-dependent antigens, but both pathways include The prototypic TI-1 antigen is lipopolysaccharide(LPS), a signals generated when multivalent antigen binds and cross- major component of the cell walls of gram-negative bacteria. links mlg(Figure 11-6). Once the B cell has acquired an ef- At low concentrations, LPS stimulates the production of fective competence signal in early activation, the interaction antibodies specific for LPS. At high concentrations, it is a of cytokines and possibly other ligands with the B-cell mem- lyclonal B-cell activator. brane receptors provides progression signalsrearrangement of their endogenous light-chain DNA. Some of these cells succeed in replacing the light chain of the self￾antigen reactive antibody with a chain encoded by endoge￾nous -chain gene segments. As a result, these cells will begin to express an “edited” mIgM with a different light chain and a specificity that is not self-reactive. These cells escape negative selection and leave the bone marrow. B-Cell Activation and Proliferation After export of B cells from the bone marrow, activation, pro￾liferation, and differentiation occur in the periphery and re￾quire antigen. Antigen-driven activation and clonal selection of naive B cells leads to generation of plasma cells and mem￾ory B cells. In the absence of antigen-induced activation, naive B cells in the periphery have a short life span, dying within a few weeks by apoptosis (see Figure 11-1). Thymus-Dependent and Thymus￾Independent Antigen Have Different Requirements for Response Depending on the nature of the antigen, B-cell activation pro￾ceeds by two different routes, one dependent upon TH cells, the other not. The B-cell response to thymus-dependent (TD) an￾tigens requires direct contact with TH cells, not simply expo￾sure to TH-derived cytokines. Antigens that can activate B cells in the absence of this kind of direct participation by TH cells are known as thymus-independent (TI) antigens. TI antigens are divided into types 1 and 2, and they activate B cells by different mechanisms. Some bacterial cell-wall components, including lipopolysaccharide (LPS), function as type 1 thymus-independent (TI-1) antigens. Type 2 thymus-independent (TI-2) antigens are highly repetitious molecules such as polymeric proteins (e.g., bacterial flagellin) or bacterial cell-wall polysaccharides with repeating polysaccharide units. Most TI-1 antigens are polyclonal B-cell activators (mito￾gens); that is, they are able to activate B cells regardless of their antigenic specificity. At high concentrations, some TI-1 antigens will stimulate proliferation and antibody secretion by as many as one third of all B cells. The mechanism by which TI-1 antigens activate B cells is not well understood. When B cells are exposed to lower concentrations of TI-1 antigens, only those B cells specific for epitopes of the antigen will be activated. These antigens can stimulate antibody pro￾duction in nude mice (which lack a thymus and thus are greatly deficient in T cells), and the response is not greatly augmented by transferring T cells into these athymic mice, indicating that TI-1 antigens are truly T-cell independent. The prototypic TI-1 antigen is lipopolysaccharide (LPS), a major component of the cell walls of gram-negative bacteria. At low concentrations, LPS stimulates the production of antibodies specific for LPS. At high concentrations, it is a polyclonal B-cell activator. TI-2 antigens activate B cells by extensively crosslinking the mIg receptor. However, TI-2 antigens differ from TI-1 antigens in three important respects. First, they are not B-cell mitogens and so do not act as polyclonal activators. Second, TI-1 antigens will activate both mature and immature B cells, but TI-2 antigens activate mature B cells and inactivate im￾mature B cells. Third, although the B-cell response to TI-2 antigens does not require direct involvement of TH cells, cytokines derived from TH cells are required for efficient B-cell proliferation and for class switching to isotypes other than IgM. This can be shown by comparing the effect of TI-2 antigens in mice made T-cell–deficient in various ways. In nude mice, which lack thymus-derived T cells but do contain a few T cells that arise from other sites that probably lie in the intestine, TI-2 antigens do elicit B-cell responses. TI-2 anti￾gens do not induce antibody production in mice that cannot express either  or  TCRs because the genes encoding the TCR  and  chains have been knocked out. Administration of a few T cells to these TCR-knockout mice restores their ability to elicit B-cell responses to TI-2 antigens. The humoral response to thymus-independent antigens is different from the response to thymus-dependent antigens (Table 11-2). The response to TI antigens is generally weaker, no memory cells are formed, and IgM is the predominant antibody secreted, reflecting a low level of class switching. These differences highlight the important role played by TH cells in generating memory B cells, affinity maturation, and class switching to other isotypes. Two Types of Signals Drive B Cells into and Through the Cell Cycle Naive, or resting, B cells are nondividing cells in the G0 stage of the cell cycle. Activation drives the resting cell into the cell cy￾cle, progressing through G1 into the S phase, in which DNA is replicated. The transition from G1 to S is a critical restriction point in the cell cycle. Once a cell has reached S, it completes the cell cycle, moving through G2 and into mitosis (M). Analysis of the progression of lymphocytes from G0 to the S phase revealed similarities with the parallel sequence in fi￾broblast cells. These events could be grouped into two cate￾gories, competence signals and progression signals. Compe￾tence signals drive the B cell from G0 into early G1, rendering the cell competent to receive the next level of signals. Pro￾gression signals then drive the cell from G1 into S and ulti￾mately to cell division and differentiation. Competence is achieved by not one but two distinct signaling events, which are designated signal 1 and signal 2. These signaling events are generated by different pathways with thymus-independent and thymus-dependent antigens, but both pathways include signals generated when multivalent antigen binds and cross￾links mIg (Figure 11-6). Once the B cell has acquired an ef￾fective competence signal in early activation, the interaction of cytokines and possibly other ligands with the B-cell mem￾brane receptors provides progression signals. 254 PART II Generation of B-Cell and T-Cell Responses