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Mulberry mosaic dwarf (MMD) disease was first recorded in 1957 and has long been endemic in China. A new viroid-like RNA was detected from mulberry trees affected by MMD and its full nucleotide sequence was determined (1). This mulberry small circular RNA (mscRNA), consisting of 356 nt, does not share significant sequence homology with other viroids and viroid-like satellite RNAs. Here we show that (+) strand of mcRNA contains a hammerhead ribozyme, while the (-) strand contains a hairpin ribozyme, resembling viroid-like satellite RNAs of nepoviruses. mscRNA transcripts of (+) and (-) strands self-cleaved in vitro as predicted by the hammerhead and hairpin structures, respectively, generating fragments with the expected 5' terminus. Circular forms of mscRNA, purified from plant extracts, and dimeric transcripts of both polarities, generated in vitro, were slash-inoculated in mulberry, hop, tomato and cucumber seedlings. Northern-blot hybridization did not detect mscRNA in the inoculated plants so far, suggesting that this circular RNA is not endowed of autonomous replication. These data, taking also into consideration that a nepovirus has been recently reported in mulberry in China (2), strongly suggest that mscRNA could be a novel viroid-like satellite RNA of a nepovirus. This hypothesis is supported by the identification, by next generation sequencing, of both mscRNA and a nepovirus in the same mulberry host. Moreover, a close association of mscRNA with MMD disease was excluded on the bases of a field survey carried out on mulberry trees affected or not affected by the disease. This data are in agreement with our recent results showing that MMD is closely associated with a new geminivirus (3).

Nef is one of the non-structural genes encoded by human immunodeficiency viruses. The protein product is associated with the cellular and plasma membranes, and is synthesized soon after entry of the virus. However, little is known about its functions relevant to viral replication and cytopathogenesis. CD4 is considered to be a crucial molecule for the signal transduction and maturation of T cells, and also serves as the receptor for HIV-1. It has been suggested that the down-regulation of cell surface CD4 by Nef proteins is somehow controversial. In order to evaluate the effects of Nef on the CD4 molecule, we constructed a CD4+ monocytoid cell line in which the HIV-1 nef gene was placed under the transcriptional control of the human metallothionein IIA promoter. The analysis of this cellular clone showed that CD4 on the cell surface was down-regulated when Nef proteins were induced. The amount of CD4 antigens on the cell surface correlated inversely with the level of Nef protein expression. This down-regulation of CD4 antigens by Nef was found to occur at the post-translational level. These results showed that the nef gene could modulate the pathophysiology of AIDS by altering the surface CD4 expression.

Using KOLT-2 monoclonal antibody (mAb) recognizing a 44,000 molecular weight (MW) T-cell activation antigen (CD28), we observed the co-expression of KOLT-2 (CD28) antigen and Tac (CD25) antigen, associated with a 55,000 MW chain of the interleukin-2 receptor (IL-2R) complex. IL-2R (p55), on several T-cell lines transformed by human T-lymphotropic virus-I (HTLV-I), as well as several subclones of the natural killer (NK)-like cell line YT. When YT cells (YTC3T, YT5.1) and IL-2-dependent HTLV-I+ T-cell line cells (ED, ATL35C) expressing both IL-2R (p55) and IL-2R (p70) chains were cultured with recombinant IL-2 (rIL-2), KOLT-2 antigen was down-regulated in 24 hr. However, KOLT-2 antigen on HPB-ALL (Kurume) cells expressing neither of the IL-2R chains was unaffected by IL-2. IL-2 also failed to down-regulate KOLT-2 antigen on MT-1 cells bearing IL-2R (p55)/Tac without IL-2R (p70). To clarify the role of IL-2R (p70) in the IL-2-induced down-regulation of KOLT-2 antigen, we analysed the effect of IL-2 on a Tac- subclone of YT (YT2C2) that expresses IL-2R (p70). As was the case with Tac+ YT cells, KOLT-2 antigen was down-regulated by IL-2, and this down-regulation was inhibited by anti-IL-2R (p70) mAb but not by anti-Tac mAb. These results show that the signalling through the IL-2/IL-2R system down-regulates KOLT-2 (CD28) antigen by way of the interaction between IL-2 and IL-2R (p70), irrespective of IL-2R (p55)/Tac. Possible involvement of the IL-2/IL-2R system in the CD28-mediated mitogenic mechanism is discussed.