IL-6 transgenic mouse model for extraosseous plasmacytoma

Abstract

Plasma cell neoplasms in humans comprise plasma cell myeloma, otherwise known as multiple myeloma, Ig deposition and heavy chain diseases, and plasmacytoma (PCT). A subset of PCT, designated extramedullary PCT, is distinguished from multiple myeloma and solitary PCT of bone by its distribution among various tissue sites but not the bone marrow. Extramedullary (extraosseus) PCT are rare spontaneous neoplasms of mice but are readily induced in a susceptible strain, BALB/c, by treatment with pristane. The tumors develop in peritoneal granulomas and are characterized by Myc-activating T(12;15) chromosomal translocations and, most frequently, by secretion of IgA. A uniting feature of human and mouse plasma cell neoplasms is the critical role played by IL-6, a B cell growth, differentiation, and survival factor. To directly test the contribution of IL-6 to PCT development, we generated BALB/c mice carrying a widely expressed IL-6 transgene. All mice exhibited lymphoproliferation and plasmacytosis. By 18 months of age, over half developed readily transplantable PCT in lymph nodes, Peyer's patches, and sometimes spleen. These neoplasms also had T(12;15) translocations, but remarkably, none expressed IgA. Unexpectedly, approximately 30% of the mice developed follicular and diffuse large cell B cell lymphomas that often coexisted with PCT. These findings provide a unique model of extramedullary PCT for studies on pathogenesis and treatment and suggest a previously unappreciated role for IL-6 in the genesis of germinal center-derived lymphomas.

Figure 1

Incidence and transplantability of B cell lineage tumors in C.IL-6 mice (A) and survival of mice (B). Transplantability was determined in two studies in which 8 of 12 C.IL-6 mice and 11 of 17 C.IL-6 mice were successfully transplanted. Survival was determined in a group of untreated C.IL-6 mice (n = 63) and compared with control C mice (n = 80). Reduced survival in strain C.IL-6 was significant in χ² analysis (P < 0.001).

Figure 2

Morphology of B cell lineage lymphomas arising in C.IL-6 mice. (Top) Mature plasmacytic PCT (immunostain for κ light-chains); (Middle) plasmablastic PCT (hematoxylin/eosin); (Bottom) follicular B lymphoma (hematoxylin/eosin).

Figure 3

Detection of Myc and VDJ rearrangements by Southern analysis. Corresponding pairs of donor tissues (Left) and transplanted tumors (Right) are presented. Myc rearrangements that cohybridized upon stripping and reprobing to the J_H2/Eμ probe are indicated by arrows in Upper. Corresponding VDJ fragments of matching size are also indicated by arrows in Lower. The fragment pairs represent rearrangements between Myc and the 5′ Cμ region on der 12. Asterisks denote Myc rearrangements that did not cohybridize to the J_H2/Eμ probe. These rearrangements occurred presumably 3′ of Cμ. The rectangles indicate a case in which the Myc- and VDJ-rearranged clone in the tumor-free donor tissue with PCH was sufficiently expanded to permit detection by Southern analysis. Note the presence of several expanded B cell clones (multiple VDJ rearrangements) in many tissues. GL, germ-line fragment.

Figure 4

Detection of T(12;15) by fluorescence in situ hybridization (FISH) in PCT 8184. The Myc activating product of translocation, der 12, is visualized by colocalization of probes for Myc, D_H/J_H/Cμ, and Cα. The reciprocal product of translocation, der 15, is visualized by the D_H/J_H/Cμ signal. Normal chromosome (Chr) 15 (one copy) and Chr 12 (two copies) are visualized by Myc and Cα signals, respectively.

Figure 5

Map of T(12;15) breakpoints in IgH. Break sites with Myc are indicated by arrows. Each arrow represents a distinct T(12;15)⁺ cell clone. Mouse numbers and tissue designations are the same as in Table 1. Upwards pointing arrows designate Myc rearrangements in hyperplastic donor tissues: blue, rearrangements occurred in tissues that were not transplantable; black, rearrangements were not redetected in recipient tumors, yet they occurred in tissues that gave rise to tumors with different Myc junctions; red, rearrangements were detected in corresponding donor and recipient tissues. Three Myc rearrangements found in recipient tumors (down-pointing black arrows labeled 1 or 2) are included to illustrate two points made in the last paragraph of Results. The Myc rearrangements of 19 additional recipient tumors (Cμ, 2 cases; Cγ1, 3 cases; Cα, 3 cases; J_H/Eμ, 11 cases) were analyzed by PCR, but were not included in the figure for clarity of presentation. Samples with hyphenated numbers contained multiple T(12;15)⁺ clones in the same tissue (e.g., three clones in the PLN sample of mouse 8293; arrows labeled 3).