Samantha Paoletti

Systematic microanatomical analysis of CXCL13 and CCL21 in situ production and progressive lymphoid organization in rheumatoid synovitis

CXCL13 and CCL21 have been functionally implicated in lymphoid tissue organization both in the upstream phases of lymphoid tissue embryogenesis and in ectopic lymphoid neogenesis in transgenic mice. Here, we analyzed the relationship between CXCL13 and CCL21 production and lymphoid tissue organization in rheumatoid synovitis as a model of a naturally occurring ectopic lymphoneogenesis. Through systematic analysis of mRNA and protein expression, we defined the microanatomical relationship between CXCL13 and CCL21 in progressive aggregational and structural phases of synovial inflammatory infiltrate. We provide the first direct in situ evidence that production of CXCL13 and CCL21 (rather than simply protein binding) is associated with inflammatory lymphoid tissue formation and development with the demonstration, in organized aggregates, of a secondary lymphoid organ‐like compartmentalization and vascular association. Notably, the presence of CXCL13 and CCL21 (protein and mRNA) was also demonstrated in non‐organized clusters and minor aggregational stages, providing evidence that their induction can take place independently and possibly upstream of T‐B compartmentalization, CD21+ follicular dendritic cell network differentiation and germinal center formation. Our data support the concept that, under inflammatory conditions, CXCL13 and CCL21 participate in lymphoid tissue microanatomical organization, attempting to recapitulate, in an aberrant lymphoid neogenetic process, their homeostatic and morphogenetic physiologic functions.

Functional CD1a is stabilized by exogenous lipids.

Self‐glycosphingolipids bind to surface CD1 molecules and are readily displaced by other CD1 ligands. This capacity to exchange antigens at the cell surface is not common to other antigen‐presenting molecules and its physiological importance is unclear. Here we show that a large pool of cell‐surface CD1a, but not CD1b molecules, is stabilized by exogenous lipids present in serum. Under serum deprivation CD1a molecules are altered and functionally inactive, as they are unable to present lipid antigens to T cells. Glycosphingolipids and phospholipids bind to, and restore functionality to CD1a without the contribution of newly synthesized and recycling CD1a molecules. The dependence of CD1a stability on exogenous lipids is not related to its intracellular traffic and rather to its antigen‐binding pockets. These results indicate a functional dichotomy between CD1a and CD1b molecules and provide new information on how the lipid antigenic repertoire is immunologically sampled.

I-TAC/CXCL11 is a natural antagonist for CCR5

The selective CXC chemokine receptor 3 (CXCR3) agonists, monokine induced by interferon‐γ (IFN‐ γ)/CXC chemokine ligand 9 (CXCL9), IFN‐inducible protein 10/CXCL10, and IFN‐inducible T cell α chemoattractant (I‐TAC)/CXCL11, attract CXCR3+ cells such as CD45RO+ T lymphocytes, B cells, and natural killer cells. Further, all three chemokines are potent, natural antagonists for chemokine receptor 3 (CCR3) and feature defensin‐like, antimirobial activities. In this study, we show that I‐TAC, in addition to these effects, acts as an antagonist for CCR5. I‐TAC inhibited the binding of macrophage‐inflammatory protein‐1α (MIP‐1α)/CC chemokine ligand 3 (CCL3) to cells transfected with CCR5 and to monocytes. Furthermore, cell migration evoked by regulated on activation, normal T expressed and secreted (RANTES)/CCL5 and MIP‐1β/CCL4, the selective agonist of CCR5, was inhibited in transfected cells and monocytes, respectively. In two other functional assays, namely the release of free intracellular calcium and actin polymerization, I‐TAC reduced CCR5 activities to minimal levels. Sequence and structure analyses indicate a potential role for K17, K49, and Q51 of I‐TAC in CCR5 binding. Our results expand on the potential role of I‐TAC as a negative modulator in leukocyte migration and activation, as I‐TAC would specifically counteract the responses mediated by many “classical,” inflammatory chemokines that act not only via CCR3 but via CCR5 as well.

Synergy‐inducing chemokines enhance CCR2 ligand activities on monocytes

The migration of monocytes to sites of inflammation is largely determined by their response to chemokines. Although the chemokine specificities and expression patterns of chemokine receptors are well defined, it is still a matter of debate how cells integrate the messages provided by different chemokines that are concomitantly produced in physiological or pathological situations in vivo. We present evidence for one regulatory mechanism of human monocyte trafficking. Monocytes can integrate stimuli provided by inflammatory chemokines in the presence of homeostatic chemokines. In particular, migration and cell responses could occur at much lower concentrations of the CCR2 agonists, in the presence of chemokines (CCL19 and CCL21) that per se do not act on monocytes. Binding studies on CCR2+ cells showed that CCL19 and CCL21 do not compete with the CCR2 agonist CCL2. Furthermore, the presence of CCL19 or CCL21 could influence the degradation of CCL2 and CCL7 on cells expressing the decoy receptor D6. These findings disclose a new scenario to further comprehend the complexity of chemokine‐based monocyte trafficking in a vast variety of human inflammatory disorders.