Follicular dendritic cells stimulated by collagen type I develop dendrites and networks in vitro

Abstract

Follicular dendritic cells (FDCs) reside in germinal centers in which their dendrites interdigitate and form non-mobile networks. FDC purification requires the use of collagenase and selection columns and leaves FDCs without detectable dendrites when examined by light microscopy. We have reasoned that isolated FDCs might reattach to a collagen matrix, extend their processes, and form immobile networks in vitro. As a test for this, cells were plated on collagen type I, laminin, biglycan, and hyaluronan. After 12 h, 80%-90% of FDCs adhered to all tested matrices but not to plastic. Within 2 weeks, FDCs adhering to type I collagen had spread out and had begun to acquire processes with occasional interconnections. By day 30, most FDCs had fine processes that formed networks through interdigitation with neighboring cells. FDC identity was confirmed by FDC-M1 labeling, immune complex trapping, and retention by FDCs in the networks. Scanning electron microscopy confirmed that groups of FDCs were in networks composed of convolutions and branching dendrites emanating from FDC cell bodies. In vivo, collagen type I was co-localized with FDCs, 5 h after challenge of immune mice with antigen. However, 2 days later, the collagen type I fibers were largely found at the periphery of the active follicles. Flow cytometry established the expression of CD29 and CD44 on FDCs; this may have partly mediated FDC-collagen interactions. Thus, we report, for the first time, that FDCs attach to collagen type I in vitro and regenerate their processes and networks with features in common with networks present in vivo.

Ultrastructural study of highly enriched follicular dendritic cells reveals their morphology and the periodicity of immune complex binding

“…Furthermore, these isolated FDCs are able to trap ICs in vitro and arrange them with a regular periodicity known to increase B cell stimulation. These findings are the latest in a series of reports (Sukumar et al 2006a, b;Aydar et al 2005;El Shikh et al 2007) demonstrating the suitability of FDCs isolated by this method for a variety of applications, including biochemical, genetic, molecular, and functional analyses. The periodicity of ICs on FDCs, initially reported in 1985 and described in more detail here, provides an additional mechanistic explanation for ICs on FDCs being more immunogenic than free antigen Aydar et al 2005).…”

“…These FDCs are also ideal for genetic and biochemical studies of FDCs (Sukumar et al 2006a;El Shikh et al 2006). In addition, we have recently reported that positively selected FDCs, when cultured on a collagen matrix, develop dendritic processes and form an extensive interdigitating reticulum (El Shikh et al 2007). We have successfully used positively selected FDCs in complex co-cultures that aim to replicate processes occurring in in-vivo GCs including somatic hypermutation, affinity maturation, and class switching (Aydar et al 2005;Wu et al 2008).…”

“…Note that the cluster of FDCs bearing ICs in c was not labeled green in d when anti-cadherin was omitted in the negative control. Bar 10 μm reticula formation can be generated in vitro if isolated FDCs are incubated on collagen (El Shikh et al 2007). We reason that cadherins may be important in establishing these cell-cell interactions.…”

Splenic CD19−CD35+B220+ cells function as an inducer of follicular dendritic cell network formation

“…that such process may initiate the transdifferentiation of stromal cells that function as initiators of FDC-network formation ( Figures 4, 6). Furthermore, this characteristic of the CD35 ϩ B220 ϩ cells can also explain the phenotypic diversity of FDCs 17,46 and can partially account for the discrepancies mentioned above.…”

Stromal cell heterogeneity in lymphoid organs

“…In contrast to the FRC, FDC have been studied mainly in connection with their immunological function [18] and their participation in forming the structure of the SLO has been largely neglected. It was shown that FDC express receptors (CD29, b1 integrin) for binding to ECM structures, including collagen, laminin and fibronectin [35]. The binding is important in the formation of a network within the B cell area; however, it was shown recently that FRC form the conduits of the B cell area.…”