No abstract

Memory B cells formed in response to microbial antigens provide immunity to later infections; however, the inability to detect rare endogenous antigen-specific cells limits current understanding of this process. Using an antigen-based technique to enrich these cells, we found that immunization with a model protein generated B memory cells that expressed isotype-switched immunoglobulins (swIg) or retained IgM. The more numerous IgM+ cells were longer lived than the swIg+ cells. However, swIg+ memory cells dominated the secondary response due to the capacity to become activated in the presence of neutralizing serum Ig. Thus, we propose that memory relies on swIg+ cells until they disappear and serum Ig falls to a low level, in which case memory resides with durable IgM+ reserves.

Early IgM+ and switched Ig+ memory B cells develop from a germinal center (GC)–independent pathway, whereas late switched memory cells are GC dependent.

Physical detection of antigen-specific CD4 T cells has revealed features of the in vivo immune response that were not appreciated from in vitro studies. In vivo, antigen is initially presented to naïve CD4 T cells exclusively by dendritic cells within the T cell areas of secondary lymphoid tissues. Anatomic constraints make it likely that these dendritic cells acquire the antigen at the site where it enters the body. Inflammation enhances in vivo T cell activation by stimulating dendritic cells to migrate to the T cell areas and display stable peptide-MHC complexes and costimulatory ligands. Once stimulated by a dendritic cell, antigen-specific CD4 T cells produce IL-2 but proliferate in an IL-2--independent fashion. Inflammatory signals induce chemokine receptors on activated T cells that direct their migration into the B cell areas to interact with antigen-specific B cells. Most of the activated T cells then die within the lymphoid tissues. However, in the presence of inflammation, a population of memory T cells survives. This population is composed of two functional classes. One recirculates through nonlymphoid tissues and is capable of immediate effector lymphokine production. The other recirculates through lymph nodes and quickly acquires the capacity to produce effector lymphokines if stimulated. Therefore, antigenic stimulation in the presence of inflammation produces an increased number of specific T cells capable of producing effector lymphokines throughout the body.

The initial step in a humoral immune response involves the acquisition of antigens by B cells via surface immunoglobulin. Surprisingly, anatomic studies indicate that lymph-borne proteins do not have access to the follicles where naive B cells reside. Thus, it is unclear how B cells acquire antigens that drain to lymph nodes. By tracking a fluorescent antigen and a peptide:MHC II complex derived from it, we show that antigen-specific B cells residing in the follicles acquire antigen within minutes of injection, first in the region closest to the subcapsular sinus where lymph enters the lymph node. Antigen acquisition, presentation, and subsequent T cell-dependent activation did not require B cell migration through the T cell area or exposure to dendritic cells. These results indicate that the humoral response is initiated as soluble antigens diffuse directly from lymph in the subcapsular sinus to be acquired by antigen-specific B cells in the underlying follicles.

The adoptive transfer of naive CD4+ T cell receptor (TCR) transgenic T cells was used to investigate the mechanisms by which the adjuvant lipopolysaccharide (LPS) enhance T cell clonal expansion in vivo. Subcutaneous administration of soluble antigen (Ag) resulted in rapid and transient accumulation of the Ag-specific T cells in the draining lymph nodes (LNs), which was preceded by the production of interleukin (IL)-2. CD28-deficient, Ag-specific T cells produced only small amounts of IL-2 in response to soluble Ag and did not accumulate in the LN to the same extent as wild-type T cells. Injection of Ag and LPS, a natural immunological adjuvant, enhanced IL-2 production and LN accumulation of wild-type, Ag-specific T cells but had no significant effect on CD28-deficient, Ag-specific T cells. Therefore, CD28 is critical for Ag-driven IL-2 production and T cell proliferation in vivo, and is essential for the LPS-mediated enhancement of these events. However, enhancement of IL-2 production could not explain the LPS-dependent increase of T cell accumulation because IL-2–deficient, Ag-specific T cells accumulated to a greater extent in the LN than wild-type T cells in response to Ag plus LPS. These results indicate that adjuvants improve T cell proliferation in vivo via a CD28-dependent signal that can operate in the absence of IL-2.

Recently, static and dynamic imaging methods have produced the first glimpses of the interactions between antigen-specific T cells and peptide-MHC-bearing antigen-presenting cells in the lymph nodes. Using data from these experiments, we produced a numerically, spatially, and temporally scaled simulation of the first 50 hr of the primary T cell-dependent immune response. The simulation highlights how lymph node structure facilitates antigen presentation to rare, naive, antigen-specific CD4+ T cells.

Adoptive transfer of TCR-transgenic T cells uniformly expressing an identifiable TCR of known peptide/MHC specificity can be used to monitor the in vivo behavior of antigen-specific T cells. We have used this system to show that naive T cells are initially activated within the T-cell zones of secondary lymphoid tissue to proliferate in a B7-dependent manner. If adjuvants or inflammatory cytokines are present during this period, enhanced numbers of T cells accumulate, migrate into B-cell-rich follicles, and acquire the capacity to produce IFN-gamma and help B cells produce IgG2a. If inflammation is absent, most of the initially activated antigen-specific T cells disappear without entering the follicles, and the survivors are poor producers of IL-2 and IFN-gamma. Our results indicate that inflammatory mediators play a key role in regulating the anatomic location, clonal expansion, survival and lymphokine production potential of antigen-stimulated T cells in vivo.