Antigen presentation : Influence of Cell Type and Route of Antigen Uptake

Dendritic cells (DCs), which maintain tolerance and orchestrate T cell immune responses, comprise a heterogeneous group of cells. For example, in the steady state, murine spleen contains pre-DC-derived CD8+DEC-205+ and CD8-DCIR2+ conventional DCs. To examine antigen processing and presentation in vivo, antigens were specifically targeted to CD8+ and CD8- DCs using chimeric monoclonal antibodies. We find that CD8- DCs are better than CD8- DCs for presentation of exogenous antigens onto major histocompatibility complex (MHC) class II molecules due to cell intrinsic differences. DCs are responsible for initiating T cell responses. However, during inflammation, monocytes become activated and acquire some DC-like features such as expression of CD11c, MHCII and co-stimulatory molecules, yet their role in T cell activation is still a matter of investigation. Cells similar to DCs can also be produced in vitro by culturing monocytes with granulocyte-macrophage colony-stimulating factor (GM-CSF) or bone marrow progenitors (pre-DCs) with fms-related tyrosine kinase ligand (FL). Although each of these cell types can present antigen, the relative efficiency of processing and presentation after antigen capture by different routes has not yet been systematically compared. To this end we administered OVA to various conventional DCs and activated monocytes by receptor-mediated endocytosis, pinocytosis or phagocytosis and measured antigen uptake and presentation to MHCI and MHCII restricted T cells. We find that CD8- DCs are more efficient than any other types of antigen presenting cells tested in terms of presenting antigen to MHCII restricted CD4+ T cells, irrespective of the route of antigen capture or maturation status. In contrast, both subsets of splenic DCs are equally efficient at cross-presenting antigens to CD8+ T cells. All DCs and activated monocytes cross-presented antigens delivered by receptor-mediated endocytosis and pinocytosis, albeit with different efficiencies. However monocyte-derived cells differ from DCs in that they are several orders of magnitude less efficient in presenting antigens captured by phagocytosis. DCs derived from pre-DCs are unique, processing and presenting antigens efficiently irrespective of the route of antigen capture. Our observations have significant implications for understating initiation of immune responses and vaccination strategies targeting DCs and activated monocytes

Antigen targeting to dendritic cells elicits long-lived T cell help for antibody responses

Resistance to several prevalent infectious diseases requires both cellular and humoral immune responses. T cell immunity is initiated by mature dendritic cells (DCs) in lymphoid organs, whereas humoral responses to most antigens require further collaboration between primed, antigen-specific helper T cells and naive or memory B cells. To determine whether antigens delivered to DCs in lymphoid organs induce T cell help for antibody responses, we targeted a carrier protein, ovalbumin (OVA), to DCs in the presence of a maturation stimulus and assayed for antibodies to a hapten, (4-hydroxy-3-nitrophenyl) acetyl (NP), after boosting with OVA-NP. A single DC-targeted immunization elicited long-lived T cell helper responses to the carrier protein, leading to large numbers of antibody-secreting cells and high titers of high-affinity antihapten immunoglobulin Gs. Small doses of DC-targeted OVA induced higher titers and a broader spectrum of anti-NP antibody isotypes than large doses of OVA in alum adjuvant. Similar results were obtained when the circumsporozoite protein of Plasmodium yoelii was delivered to DCs. We conclude that antigen targeting to DCs combined with a maturation stimulus produces broad-based and long-lived T cell help for humoral immune responses

A dynamic T cell–limited checkpoint regulates affinity-dependent B cell entry into the germinal center

Entry into the germinal center requires antigen-bearing B cells to compete for cognate T cell help at the T–B border

Route of Antigen Uptake Differentially Impacts Presentation by Dendritic Cells and Activated Monocytes

Dendritic cells (DCs), which maintain tolerance and orchestrate T cell immune responses, comprise a heterogeneous group of cells. For example, in the steady state, murine spleen contains pre-DC-derived CD8(+) and CD8(−) conventional DCs. During inflammation, monocytes become activated and acquire some DC-like features such as expression of CD11c and MHCII. Although each of these cell types can present antigen, the relative efficiency of processing and presentation after antigen capture by different routes has not yet been systematically compared. To this end we administered OVA to various conventional DCs and activated monocytes by receptor-mediated endocytosis, pinocytosis or phagocytosis and measured internalization and presentation to MHCI and MHCII restricted T cells. We find that CD8(−) DCs are more efficient than any other type of antigen presenting cell tested in terms of presenting antigen to MHCII restricted T cells, irrespective of the route of antigen capture. In contrast both subsets of splenic DCs are highly effective in cross-presenting antigens to CD8(+) T cells. DCs and activated monocytes cross-presented antigens delivered by DEC205-mediated endocytosis and pinocytosis. However, DCs differ from activated monocytes in that the latter are several orders of magnitude less efficient in presenting antigens captured by phagocytosis to CD8(+) or CD4(+) T cells. We conclude that DCs derived from pre-DCs differ from monocyte-derived cells in that DCs process and present antigens efficiently irrespective of the route of antigen capture. Our observations have significant implications for understanding initiation of immune responses and vaccination strategies targeting DCs and activated monocytes