Inhibition of syncytia-inducing (SI) virus by autologous serum from HIV-1-infected individuals

Abstract Background: Progression from HIV infection to AIDS is often accompanied or even predicted by a switch of the virus to a more pathogenic or syncytia-inducing (SI) phenotype concomitant with the development of HIV variants escaping neutralizing antibodies. Objective: Here we studied the capacity of sera to neutralize autologous SI-HIV or the laboratory strain III B and compared these data to the viral load in HIV-1-infected patients. Methods: The SI phenotype of HIV was detected by co-cultivation of peripheral blood mononuclear cells (PBMCs) with MT2 cells in 112 patients stratified by their CD4 cell counts. Sera at dilutions of 1 : 15 and 1 : 75 were added to MT2 co-cultures with autologous PBMCs as well as with HIV-1 III B -infected H9 cells to study the inhibitory capacity. The p24 antigenemia was detected by enzyme-linked immunosorbent assay (ELISA) and the circulating HIV RNA was determined using the polymerase chain reaction (PCR). Results: The SI virus was detected in PBMCs from 31 65 patients with ⩽ 200 CD4 + cells, 8 28 patients with 201–499 CD4 + cells, and 1 19 patients with ⩾ 500 CD4 + cells. Sera from 16 40 patients inhibited the autologous SI-HIV. In sera from patients with ⩽ 200 CD4 + cells, p24 antigen could be detected in 17 34 (50%) patients with non-syncytia-in-ducing (NSI) phenotype and in 7 19 (37%) patients carrying SI-HIV without serum inhibition. In contrast, all 12 sera with inhibitory activity to the autologous SI-HIV were negative for p24 antigen. A similar tendency was seen in patients with higher CD4 + T-cell counts. The mean load of circulating HIV RNA did not differ among groups of patients. Independently of their neutralizing activity to the autologous SI virus, the majority of sera were able to neutralize the laboratory HIV-1 III B . Conclusions: While most of the patients' sera neutralized the laboratory HIV-1 III B strain, only some sera were able to inhibit the autologous SI-HIV. In these cases, the detectable SI-HIV may still be controlled by the immune system in vivo, which is consistent with a low p24 antigenemia.