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Background:  Tat has a central role in HIV replication and levels of antibodies to Tat in HIV-1-infected subjects correlate with lower virus loads and non-progression. Tat-directed immunotherapy might, thus, be useful in controlling HIV infection. Antibody blockade of circulating Tat would create no selective pressure so that resistant strains are unlikely to develop after treatment. Two conserved epitopes, aa 4 to 12 and aa 43 to 50 of the HIV-1 Tat protein have been identified as suitable as targets for therapeutic antibodies. Fully human IgG₁ antibodies recognising each epitope have been derived from the n-CoDeR scFv phage display library and we now show them to inhibit viral replication in vitro in both Jurkat cells and human peripheral blood mononuclear cells (PBMC).

Methods:  Monoclonal antibodies were selected from the scFv library using peptides covering the epitope variations. ELISA and Biacore were used to analyze binding characteristics. Jurkat cells were infected with HIV-1_IIIB and the anti-Tat antibodies were titrated for suppression of viral replication. Viral levels were estimated by p24 levels 7 days post-infection. PBMC were activated by addition of HeLa expressed Tat protein prior to HIV infection, with or without prior incubation with anti-Tat antibodies, and the susceptibility to HIV-1 challenge was measured by p24 levels in the supernatant. To investigate resistance to anti-Tat, Jurkat cells were infected with HIV-1_IIIB, and passaged weekly over 10 weeks in the presence of anti-Tat. p24 levels were determined weekly. IC₅₀ was determined on viral stocks obtained after 0, 1, 5, and 10 passages.

Results:  Anti-Tat antibodies bound their target with K_Ds around 10^-9 to 10^-11 M, as measured by surface plasmon resonance. In HIV-infected Jurkat cells viral replication or HIV-1 p24 expression was inhibited dose dependently with IC₅₀ values of 0.1 to 0.4 µg/mL. Anti-Tat antibodies inhibited Tat-induced permissivity for HIV-1 infection in PBMC. IC₅₀ values from 10-week passages show no reduction in viral sensitivity to anti-Tat suppression.

Conclusions:  Anti-Tat may be a potential therapy for HIV-1 infection, with little likelihood of the development of drug resistance.

Keywords: anti-Tat; viral inhibition; no resistance