Mechanistic medicine: novel strategies for clinical trials.

Autoimmune diseases affect a significant proportion of the population and the development of therapeutics able to manipulate the immune response to deliver effective treatment in these diseases is an accepted approach for drug discovery. This article will focus on recent strategies for achieving selectivity through target choice, thus reducing overall clinical immunosuppression. We review the use of mechanistic pharmacodynamic assays preclinically and in the clinic to assess target engagement and to build the relationship between target coverage and efficacy, to guide dosing. Finally, we review the use of monogenic diseases to deliver proof of mechanism clinical studies and to identify patient populations in larger autoimmune diseases that may be sensitive to intervention with a specific therapeutic.

The humoral immune response is initiated in lymph nodes by B cells that acquire soluble antigen directly in the follicles.

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.

CD4+ T cells that enter the draining lymph nodes after antigen injection participate in the primary response and become central-memory cells.

We explored the relationship between the time of naive CD4+ T cell exposure to antigen in the primary immune response and the quality of the memory cells produced. Naive CD4+ T cells that migrated into the skin-draining lymph nodes after subcutaneous antigen injection accounted for about half of the antigen-specific population present at the peak of clonal expansion. These late-arriving T cells divided less and more retained the central-memory marker CD62L than the T cells that resided in the draining lymph nodes at the time of antigen injection. The fewer cell divisions were related to competition with resident T cells that expanded earlier in the response and a reduction in the number of dendritic cells displaying peptide-major histocompatibility complex (MHC) II complexes at later times after antigen injection. The progeny of late-arriving T cells possessed the phenotype of central-memory cells, and proliferated more extensively during the secondary response than the progeny of the resident T cells. The results suggest that late arrival into lymph nodes and exposure to antigen-presenting cells displaying lower numbers of peptide-MHC II complexes in the presence of competing T cells ensures that some antigen-specific CD4+ T cells divide less in the primary response and become central-memory cells.

Visualizing the first 50 hr of the primary immune response to a soluble antigen.

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.

The alpha 1 beta 1 and alpha E beta 7 integrins define a subset of dendritic cells in peripheral lymph nodes with unique adhesive and antigen uptake properties.

Dendritic cells (DCs) are a heterogeneous population of APCs with critical roles in T cell activation and immune regulation. We report in this study the identification and characterization of a novel subset of DCs resident in skin-draining peripheral lymph nodes of normal mice. This subset of CD11c(high)CD40(high)CD8alpha(intermediate (int)) DCs expresses the collagen-binding integrin, alpha1beta1, and the E-cadherin-binding integrin, alphaEbeta7. Although alpha1beta1 and alphaEbeta7 are also expressed on CD11c(high)CD40(int)CD8alpha(high) lymphoid DCs, CD11c(high)CD40(high)CD8alpha(int) DCs demonstrate preferential integrin-mediated adhesion to collagen and fibronectin. This DC subset most likely acquires expression of these integrins in peripheral lymph node, as this subset is not found in the spleen or mesenteric lymph node, and recent DC migrants from the skin lack expression of alpha1beta1 and alphaEbeta7 integrins. Resident CD40(high) DCs express alpha1beta1 integrin and colocalize with collagen in lymph nodes. When compared with CD11c(high)CD40(high)CD8alpha(int) DCs lacking expression of these integrins, the alpha1beta1+alphaEbeta7+DC subset exhibits more efficient formation of Ag-independent conjugates with T cells, and a decreased ability to acquire soluble Ag. Thus, the alpha1beta1 and alphaEbeta7 integrins define a unique population of peripheral lymph node-derived DCs with altered functional properties and adhesive potential that localizes these cells to sites in lymph nodes where Ag presentation to T cells occurs.

Distinct dendritic cell populations sequentially present antigen to CD4 T cells and stimulate different aspects of cell-mediated immunity.

Peptide:MHC II complexes derived from a fluorescent antigen were detected in vivo to identify the cells that present subcutaneously injected antigen to CD4 T cells. Skin-derived dendritic cells (DCs) that acquired the antigen while in the draining lymph nodes were the first cells to display peptide:MHC II complexes. Presentation by these cells induced CD69, IL-2 production, and maximal proliferation by the T cells. Later, DCs displaying peptide:MHC II complexes migrated from the injection site via a G protein-dependent mechanism. Presentation by these migrants sustained expression of the IL-2 receptor and promoted delayed type hypersensitivity. Therefore, presentation of peptide:MHC II complexes derived from a subcutaneous antigen occurs in two temporally distinct waves with different functional consequences.

Antigen presentation to naive CD4 T cells in the lymph node.

Although the presentation of peptide-major histocompatibility complex class II (pMHC class II) complexes to CD4 T cells has been studied extensively in vitro, knowledge of this process in vivo is limited. Unlike the in vitro situation, antigen presentation in vivo takes place within a complex microenvironment in which the movements of antigens, antigen-presenting cells (APCs) and T cells are governed by anatomic constraints. Here we review developments in the areas of lymph node architecture, APC subsets and T cell activation that have shed light on how antigen presentation occurs in the lymph nodes.