In Brief

Regulatory T cells

Toll-like receptor 2 signaling modulates the function of CD4⁺CD25⁺ regulatory T cells. Liu, H. et al. Proc. Natl Acad. Sci. USA 103, 7048–7053 (2006)

Given that there are numerous regulatory T-cell populations that suppress T-cell responses, how does successful protective immunity occur in the face of such suppression? Foo Liew and colleagues report that, in a similar manner to effector T cells, naturally occurring regulatory T (T_Reg) cells express Toll-like receptor 2 (TLR2) and undergo marked proliferation in the presence of the TLR2 ligand bacterial lipoprotein (BLP) and CD3-specific antibody. Importantly, BLP-induced T_Reg-cell proliferation resulted in a transient loss of suppressive activity and forkhead box P3 (FOXP3) expression. In addition, interleukin-2 produced by BLP-activated effector T cells rendered these cells refractory to T_Reg-cell-mediated suppression. But, 3 days after BLP-mediated activation, the T_Reg cells regained FOXP3 expression and suppressive activity, as indicated by their ability, following adoptive transfer, to prevent colitis in immunodeficient mice.

Antigen-presenting cells

CD1a⁺ antigen-presenting cells in human dermis respond rapidly to CCR7 ligands. Angel, C. E. et al. J. Immunol. 176, 5730–5734 (2006)

Populations of antigen-presenting cells (APCs) that are distinct from Langerhans cells have recently been identified in mouse skin. Now, a similar population of APCs has been found in human skin. Angel et al. describe a population of APCs that express intermediate levels of CD1a⁺ but not the Langerhans-cell marker CD207 and that is located close to the lymphatic vessels in the upper layers of the dermis. They termed these cells CD1a⁺ dermal APCs. This population could prime naive CD4⁺ T cells, owing to expression of MHC class II and co-stimulatory molecules. In a similar manner to Langerhans cells, the CD1a⁺ dermal APCs expressed CC-chemokine receptor 7 (CCR7) and could migrate in response to CCR7 ligands, which are known to mediate the migration of cells from the skin to the draining lymph nodes.

Macrophages

Carbohydrate-independent recognition of collagens by the macrophage mannose receptor. Martinez-Pomares, L. et al. Eur. J. Immunol. 36, 1074–1082 (2006)

A hallmark of alternatively activated macrophages is expression of the mannose receptor, which belongs to a family of four endocytic receptors with a common domain structure. The mannose receptor has two known, distinct lectin (carbohydrate-binding) activities, which are mediated by its cysteine-rich domain and multiple C-type lectin-like domains (CTLDs). Located between these two domains is the fibronectin type II (FNII) domain, which is conserved among all mannose-receptor-family members and is important for collagen binding. Martinez-Pomares et al. examined whether the mouse mannose receptor could bind collagen, or if the CTLD- and cysteine-rich-domain-mediated lectin activity was favoured. They found that the mannose receptor could indeed recognize and internalize collagen in a carbohydrate-independent manner, and that the FNII domain mediated this function. This paper describes a third distinct ligand-binding site for the mannose receptor and therefore has important implications for the study of this receptor at the molecular level.