Most of the small number of cases of poliomyelitis which occur in countries where Sabin's attenuated poliovirus vaccines are used are temporally associated with administration of vaccine and involve polioviruses of types 2 and 3 (ref. 1). Recent studies have provided convincing evidence that the Sabin type 2 and 3 viruses themselves may revert to a neurovirulent phenotype on passage in man. We report here that a point mutation in the 5' noncoding region of the genome of the poliovirus type 3 vaccine consistently reverts to wild type in strains isolated from cases of vaccine-associated poliomyelitis. Virus with this change is rapidly selected on passage through the human gastrointestinal tract. The change is associated with a demonstrable increase in the neurovirulence of the virus.

Human T-cell leukaemia viruses (HTLVs) have genomic organization distinct from that of other replication-competent retroviruses, possessing four genes, gag, pol, env and chi. The unique fourth gene, chi (also referred to as lor), is located between env and the 3' long terminal repeat (LTR), encoding a protein of relative molecular mass 40,000 for HTLV-I and 37,000 for HTLV-II, located in the nucleus of infected cells. HTLV-I is the causative agent of adult T-cell leukaemia (ATL), a T-lymphocyte malignancy, while HTLV-II has been found associated with a T-cell variant of hairy cell leukaemia. Both viruses immortalize T cells in vitro. However, the mechanism of cellular transformation induced by HTLV is not known as there seems to be no common site of provirus integration in primary ATL cells and the virus contains no classical oncogene sequences. These observations have provoked speculation that the unique and strongly conserved chi protein (85% amino-acid homology between HTLV-I and -II) is involved in HTLV leukaemogenesis. Recent mutagenesis experiments in our laboratory have shown that the chi gene is essential for HTLV replication. It has also has been shown that the LTRs of HTLV and the related bovine leukaemia virus (BLV) are activated in trans in virus-infected cells, and, although such experiments did not directly demonstrate a role for the chi protein in transcriptional activation, it has been suggested that the chi protein is responsible for the transcriptional activation of the LTR and may be involved in cellular transformation. We have now developed a transient co-transfection assay which demonstrates that transcriptional activation of the HTLV LTR is mediated solely by the chi protein and that no other virus genes are required.

We have determined a major antigenic site for virus neutralization on the capsid protein VP1 of poliovirus type 3. Antigenic mutant viruses selected for resistance to individual monoclonal antibodies had point mutations concentrated in a region 277-294 bases downstream from the start of the region of viral RNA coding for VP1. These findings provide the basis for an improved understanding of the molecular basis of virus neutralization.

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The complete nucleotide sequence has been determined of a strain of poliovirus type 3, P3/119, isolated from the central nervous system of a victim of fatal vaccine-associated poliomyelitis. Comparison of this sequence with those obtained previously for the Sabin type 3 vaccine, P3/Leon 12a1b and its neurovirulent progenitor, P3/Leon/37, reveals that these three strains are on a direct geneaological lineage and therefore that P3/119 is a bona fide revertant of the vaccine. P3/119 differs in sequence from its attenuated vaccine parent at just seven positions. Only one of these differences, a mutation from U to C at position 472 in the presumed noncoding region of the genome, is a back mutation to the wild type sequence. Of the six other differences, three give rise to coding changes in virus structural proteins, two are silent changes in the major open reading frame of the genome and one affects the 3'-terminus just prior to the poly A tract. These differences indicate that there are three possible types of molecular change which could, singly or collectively, result in attenuation and reversion to neurovirulence of the Sabin type 3 vaccine.

The complete nucleotide sequence of the genome of the Sabin vaccine strain of poliovirus type 3 (P3/Leon 12 a1 b) has been determined from cDNA cloned in E. coli. The genome comprises a 5' non-coding region of 742 nucleotides, a large open reading frame of 6618 nucleotides (89% of the sequence) and a 3' non-coding region of 72 nucleotides. There is 77.4% base-sequence homology and 89.6% predicted amino-acid homology between types 1 and 3. Conservation of all glutamine-glycine and tyrosine-glycine cleavage sites suggests a mechanism of polyprotein processing similar to that established for poliovirus type 1.

Infectious recombinant viruses were constructed from three molecularly cloned human immunodeficiency virus (HIV) strains varying in cell tropism. All recombinants showed a high infectivity titer on phytohemagglutinin-stimulated normal T lymphocytes. However, a 120-bp region of the envelope gene including the area of the V3 hypervariable loop was found to influence infectivity titer on both clone 1022 CD4-positive HeLa cells and CD4-positive CEM leukemia cells. Infectivity for macrophages was more complex. All viruses replicated in macrophages to a low level, but viral sequences both inside and outside the V3 loop region influenced the efficiency of replication. Two experiments showed that the mechanism of restriction of infection of 1022 cells by HIV strain JR-CSF was related to lack of virus entry. First, productive virus infection occurred after transfection of 1022 cells with viral plasmid DNA. Second, the nonpermissive HIV strain JR-CSF could infect 1022 cells when pseudotyped with the envelope of other retroviruses, including human T-cell leukemia virus type I (HTLV-I), HTLV-II, and amphotropic murine leukemia virus. These results demonstrate the possibility that unexpected cell types might be infected with HIV in human patients coinfected with HIV and HTLV-I or HTLV-II. Human immunodeficiency virus (HIV) is known for its ability to infect CD4-positive T lymphocytes. However, HIV is also capable of infecting other cell types, including macrophages of blood and various organs (22, 39, 45), intestinal epithelial cells (2, 41), brain capillary endothelial cells (53), placental cells (38), and various cells derived from neural and connective tissues (8, 9, 13, 14, 17, 49, 51). HIV-infected macrophages have been observed in human brain tissues (28, 32, 50), and such cells may be involved in the AIDS dementia syndrome. HIV infection of macrophages and T lymphocytes has also been studied in vitro. HIV can be isolated from peripheral blood mononuclear cells (PBMCs) of most seropositive individuals by cocultivating with differentiated macrophages (23, 24) or phytohemagglutinin (PHA)stimulated CD4-positive lymphoblasts from normal donors (30). These results suggest that the HIV in most patients is either dual tropic for both macrophages and T cells or that mixed macrophage-tropic and T-cell-tropic virus populations coexist in these individuals. Some HIV stocks may show a preference for infecting macrophages rather than T lymphoblasts (22), but other cloned macrophage-tropic HIV strains infect both macrophages and T lymphoblasts (34). However, none of these macrophage-tropic HIV strains infects T-leukemia cell lines. Conversely, many HIV strains have been adapted to continuous laboratory passage in T-cell leukemia lines. These viruses can also infect PHA-stimulated T lymphoblasts but usually fail to infect macrophages (16, 24). Recent data have identified sequences in the HIV envelope

The human immunodeficiency virus type 1 (HIV-1) and human T-cell leukemia virus type I (HTLV-I) are two distinct human retroviruses that infect T cells. Recent epidemiologic studies have identified a cohort of individuals that are coinfected with both viruses. It is reported here that human peripheral blood leukocytes infected with HIV-1 in vitro can be induced to produce large quantities of HIV-1 after mitogenic stimulation by noninfectious HTLV-I virions. It is also shown that HTLV-I virions may exert this effect prior to, immediately following, or well after the cells are infected with HIV-1. These results provide further impetus for epidemiologic studies of dually infected individuals to determine whether HTLV-I may act as a cofactor for acquired immunodeficiency syndrome (AIDS).