Ectopic Lymphoid Follicle In a Mouse Experimental Autoimmune Encephalomyelitis Model

Background: Ectopic lymphoid follicles (ELFs) have been found in the meninges of patients with progressive multiple sclerosis and in the SJL mouse model of experimental autoimmune encephalomyelitis (EAE). ELFs consist of clusters of B cells organized for antigen presentation, which contain follicular dendritic cells that express CXCL13.

Objectives: To characterize ELFs in the SJL EAE mouse model and to correlate with clinical information.

Methods: Eight female SJL mice were injected subcutaneously with (0.2 mg/mouse) PLP 139–151 peptide (HCLGKWLGHPDKF) in complete Freund’s adjuvant on days 0 and 7, and with pertussis toxin (0.2 µg/mouse) intraperitoneally on days 0, 1, 7 and 8.  Clinical disease scores were recorded daily. Four mice were sacrificed at 5 weeks, and four at 15 weeks.  Sections of brain and spinal cord were immunostained for B cells (B220; BD Pharmingen), T cells (CD3; Santa Cruz), and follicular dendritic cells (CXCL13; R&D Systems). Spleen was used as a positive control. Sections from two animals were also stained for myelin proteolipid protein (PLP; Serotec) to detect areas of demyelination. Groups of ≥ 5 B220 cells defined clusters. Clusters were classified as ELFs if they contained CXCL13 expressing cells. Clusters and ELFs, along with the numbers of B cells, T cells, and CXCL13⁺dendritic cells within each, were enumerated.

Results:  Six of eight EAE-affected mice displayed ELFs. The two that did not were both sacrificed at the earlier time point of 5 weeks, although each had relapsed. Conversely, ELFs were identified in all four mice sacrificed at 15 weeks including the two mice that did not relapse.  Although B cell clusters could be detected intraparenchymally, ELFs were seen only in the meninges or around superficial vessels of the brainstem. A few ELFs were also seen in the spinal cord meninges. Using PLP staining to detect myelin, no demyelination was observed near ELFs. When observed, CXCL13 was identified within B cell aggregations, but not all B cell clusters represented ELFs. More T cells than B cells were present within clusters and ELFs.

Conclusions: We confirmed the development of ELFs in the SJL mouse model of EAE. Based on a limited sample, ELFs appeared to require chronic disease of many weeks for development. ELFs appeared to have a predilection for the ventral meninges of the brainstem, consistent with a previous report. Future work with expanded numbers of mice and further characterization is planned.