Quantitation and specificity of HIV-1 binding to HFA. HFA were exposed for 1 h at 4°C to NL4-3 alone or in the presence of anti-CCR3, anti-CCR5, or anti-CXCR4 antibodies. For comparison, CD4-negative HeLa or 293T cells or CD4-positive MAGI cells were also exposed to NL4-3. HIV-1 binding, as determined by the amount of p24 core antigen associated with cells, was measured by ELISA. Data are expressed relative to HFA.