Human immunodeficiency virus type 1 (HIV-1) infection was studied in two different human neuroblastoma cell lines, SK-N-MC and SH-SY5Y. Results from immunofluorescence analysis indicate that SK-N-MC cells express a 68K neurofilament, and SH-SY5Y cells express additionally a 160K to 200K neurofilament complex and thus represent a more differentiated state. HIV-1 infection in these cell lines was demonstrated by nested polymerase chain reaction and further characterized by hybridization, which showed that about 50% of SK-N-MC cells and 20% of SH-SY5Y cells were infected by HIV-1 and contained integrated proviral HIV-1 DNA. Among the cytokines and growth factors studied, tumour necrosis factor alpha (TNF-α) enhanced virus production in both cell lines, but to a differing extent, according to our mRNA and p24 antigen capture assay. In SK-N-MC cells the enhancement of HIV-1 mRNA was detected after 24 h of stimulation, and declined to the control level by 48 h. In SH-SY5Y cells a clear-cut stimulation was seen at both time points. By contrast, interleukin-6 (IL-6) enhanced the virus replication only in SK-N-MC cells, as shown at the mRNA level. Immunochemical staining showed no differences in the proportion of HIV-1-positive cells after 48 h of stimulation by TNF-α or IL-6 when compared to the control cells. In addition, based on a thymidine incorporation assay, TNF-α inhibited, but IL-6 strongly increased, the DNA synthesis in SK-N-MC cells, whereas in the SH-SY5Y cell line no such differences were seen. We discuss the possibility that developing, less-differentiated neurons may be more readily infected by HIV-1 than fully differentiated neurons, and that cytokines such as TNF-α and IL-6, which are elevated in HIV-1-infected individuals, may enhance HIV production.


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