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Polyclonal B cell activation induced by herpesvirus saimiri-transformed human CD4+ T cell clones. Role for membrane TNF-alpha/TNF-alpha receptors and CD2/CD58 interactions.

We have shown that in vitro infection herpesvirus saimiri (HVS) can transform human CD4+ T cell clones with defined Th1 or Th2 cytokine profiles to continuous growth. We report here that transformation with HVS enabled both Th1 and Th2 clones to stimulate proliferation and Ig production by autologous or allogeneic B cells in the absence of stimulants. The polyclonal B cell-activating property of HVS-transformed clones was not related to free virus or soluble cytokines, but rather was dependent on an Ag-nonspecific, MHC-unrestricted, contact-dependent mechanism. T blasts from unstimulated HVS-transformed clones did not express CD40 ligand (CD40L) mRNA or CD40L protein, whereas a proportion of them constitutively expressed membrane TNF (mTNF)-alpha. Both CD40L and mTNF-alpha were detectable on either uninfected or HVS-transformed clones upon mitogen stimulation. The activation of high-density B cells by unstimulated HVS-transformed clones was not inhibited by soluble CD40-Ig fusion protein, but was strongly reduced by either anti-TNF-alpha or anti-TNF-alpha receptor (TNF-alpha R) mAbs. Addition of anti-CD2 and/or anti-CD58 mAbs was also inhibitory, but no additive effect with anti-TNF-alpha and/or anti-TNF-alpha R mAbs was observed. Neither anti-IL-2 nor CD40-Ig inhibited the proliferation of naive IgD+ B cells cocultured with fixed unstimulated HVS-transformed clones, whereas a combination of anti-TNF-alpha and anti-TNF-alpha R mAbs was inhibitory. In addition, fixed unstimulated HVS-transformed clones induced Ig synthesis in IgD+ naive B cells even in the absence of exogenous IL-2. Data suggest that both the mTNF-alpha/TNF-alpha R and the CD2/CD58 pathways, but not the CD40L-CD40 interaction plus secreted cytokines, are involved in the unusual mode of B cell activation exerted by CD4+ HVS-transformed clones.