Tpl2^−/− monocytes/macrophages display defective inflammatory switch in the myeloma microenvironment. BM aspirates from Vκ*MYC/Tpl2^+/+ or Vκ*MYC/Tpl2^−/− animals were collected, and monocytic-lineage cells were isolated by CD115⁺ immunomagnetic separation. Complementary DNA subjected to quantitative RT-PCR using primers recognizing various cytokines as indicated. (A) CD115⁺ cells from 3-month-old Vκ*MYC mice (0 incidence of M-spikes) do not transcribe appreciable amounts of IL-1β message. By 8 months, most Vκ*MYC mice have developed M-spikes. CD115⁺ cells at 8 months transcribe high levels of IL-1β message, demonstrating an inflammatory switch equivalent to the induction of inflammatory mediators reported by our group in human MAM.²⁴  (B) At 3 months of age, Vκ*MYC mice lack monoclonal gammopathy (0 incidence of M-spikes, see Figure 4A). IL-1β message is transcribed at the same low rates regardless of Tpl2 status. (C) By 8 months of age, CD115⁺ from Vκ*MYC/Tpl2^+/+ mice have undergone inflammatory switch, characterized by increased IL-1β production. However, CD115⁺ cells from Vκ*MYC/Tpl2^−/− mice display severe defects in IL-1β and IL-6 production. (D) The defect in inflammatory cytokine production by Tpl2^−/− MAM persists even in overt tumor (tumor-matched mice bearing equivalent M-spikes). P values: *<.05, **<.01, ***<.001. mRNA, messenger RNA.