The susceptibility of neonates to virus-induced disease is thought to reflect, in part, the immaturity of their immune systems. However, inoculation of newborn mice with low doses of Cas-Br-M murine leukemia virus induced a protective cytotoxic T lymphocyte (CTL) response. The inability of neonates to develop a CTL response to high doses of virus was not the result of immunological immaturity but correlated with the induction of a nonprotective type 2 cytokine response. Thus, the initial viral dose is critical in the development of protective immunity in newborns.

The pathogenesis of progressive spastic paraparesis [HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP)], a serious consequence of human T-cell leukemia virus type I (HTLV-I) infection, is unclear. T and B lymphocytes can be naturally infected by HTLV-I, but the susceptibility to HTLV-I infection of other cell types that could contribute to the pathogenesis of HAM/TSP has not been determined. We found that a human monocyte cell line (T]HP-1), primary human peripheral blood monocytes, and isolated microglial cells but not astrocytes or oligodendroglial cells derived from adult human brain were infected by HTLV-I in vitro. Infection with HTLV

Tropical spastic paraparesis (HAM/TSP) is caused by human T-cell lymphotropic virus type I (HTLV-I) infection. Although the virus infects T cells in vivo and is capable of infecting microglia in vitro, the inflammatory demyelination has not been linked to virus in central nervous system tissue. Thus, indirect mechanisms (e.g., cytokines) could be involved in demyelination and inflammation. The ability of HTLV-I Tax protein to induce tumor necrosis factor-alpha (TNF-alpha), interleukin-1 beta (IL-1 beta), and IL-6 in primary adult human microglia and peripheral blood macrophages (PBMs) was examined by enzyme-linked immunosorbent assay and reverse transcription-polymerase chain reaction (RT-PCR). Tax (20 ng/ml) induced TNF-alpha in microglia (from undetectable or low basal levels to 215-1,075 pg/ml, mean 576 +/- 375 pg/ml, n = 4) and in PBMs (70-1,900, mean 646 +/- 844 pg/ml, n = 4). This induction was dose dependent, Tax specific, and maximal at 8 hours after stimulation. IL-6 levels in microglia increased from a basal level of 368 +/- 194 to 664 +/- 270 pg/ml 24 hours after Tax stimulation. In contrast, IL-1 beta levels were modestly induced (< or = 26 pg/ml). An increase in mRNA levels of the three cytokines was observed by semiquantitative RT-PCR (TNF-alpha = 28-fold; IL-6 = 5.6-fold; IL-1 beta = 3.6-fold). Thus, in HAM/TSP, extracellular Tax released from infiltrating T cells could induce cytokine release by microglia and contribute to demyelination and inflammation in the absence of detectable virus.

PVC-211 murine leukemia virus (MuLV) is a neuropathogenic variant of Friend MuLV (F-MuLV) that causes a rapidly progressive neurodegenerative disease in susceptible rodents. PVC-211 MuLV, but not the parental F-MuLV, can infect rat brain capillary endothelial cells (BCEC) efficiently, and the major determinant for BCEC tropism of PVC-211 MuLV is localized within the XbaI-BamHI fragment of the viral genome containing the 5' half of the env gene. To further dissect the XbaI-BamHI region for its effects on BCEC tropism, we constructed recombinant viruses between PVC-211 MuLV and F-MuLV and tested their infectivity on a cell line established from rat BCEC. Our results indicated that Glu116-to-Gly and Glu129-to-Lys substitutions in the background of the F-MuLV envelope SU protein were sufficient for conferring BCEC tropism on the virus. Interference studies of these viruses on Rat-1 fibroblastic cells showed that the structure of the SU protein encoded by the XbaI-BamHI region also has significant effects on their affinity for the rat ecotropic MuLV receptor. These results support the possibility that structural elements I and II of the SU protein are important determinants for virus-receptor interaction.

PVC-211 murine leukemia virus (MuLV) is a replication-competent, ecotropic type C retrovirus that was isolated after passage of the Friend virus complex through F344 rats. Unlike viruses in the Friend virus complex, it does not cause erythroleukemia but causes a rapidly progressive hind limb paralysis when injected into newborn rats and mice. We have isolated an infectious DNA clone (clone 3d) of this virus which causes neurological disease in animals as efficiently as parental PVC-211 MuLV. The restriction map of clone 3d is very similar to that of the nonneuropathogenic, erythroleukemogenic Friend murine leukemia virus (F-MuLV), suggesting that PVC-211 MuLV is a variant of F-MuLV and that no major structural alteration was involved in its derivation. Studies with chimeric viruses between PVC-211 MuLV clone 3d and wild-type F-MuLV clone 57 indicate that at least one determinant for neuropathogenicity resides in the 2.1-kb XbaI-ClaI fragment * Corresponding author. PVC-211 MuLV plays no significant role in neuropathogenicity but may be responsible for the failure of this virus to cause erythroleukemia. MATERIALS AND METHODS Cells and viruses. PVC-211 MuLV-producing normal rat kidney (NRK) cells (18) were obtained from B. Pogo, The Mount Sinai Medical Center, New York, N.Y., with the permission of K. Kai, Yamaguchi University, Yamaguchi, Japan. Supernatant from these cells was used to infect NIH 3T3 cells, and the virus-producing culture (104 to 105 XC-PFU/ml) was used for these studies. NIH 3T3 cells transfected with an infectious DNA clone (clone 57 [25]) of F-MuLV was used as a producer of wild-type F-MuLV (105 to 106 XC-PFU/ml). All the cells were grown on Dulbecco's modified Eagle's medium supplemented with 10% fetal calf serum. Virus titer, infectivity, and host range were determined by XC cell fusion (33), mink S+Lcell focus induction (27), and reverse transcriptase (19) assays. Animals. Although PVC-211 MuLV also causes neurological disease in NFS/N mice (15), the disease had a more rapid onset and progression in F344 rats. Therefore, the ability of viruses to cause neurological disease was assessed by using newborn F344 rats obtained from Harlan Sprague Dawley, Indianapolis, Ind., and housed in the Small Animal Facility,