The major structural proteins of most viruses, including both naked icosahedral and enveloped types, are present in a dense array on the virion surface. This pattern has likely evolved to promote structural integrity, maximize cell binding and entry, and minimize genome size. HIV and related simian lentiviruses are unusual in having a low density of envelope protein spikes on their surfaces. Why has HIV evolved this unusual virion structure?
This discussion paper suggests that having low numbers of envelope spikes retards the induction of a broad spectrum antibody response. Neutralizing antibodies are exceptionally slow to develop during HIV infection, and nearly all broadly neutralizing antibodies invariably have undergone a large number of hypersomatic mutations. Artificial virus-like particles (VPLs) with a high density of envelope spikes might make a safe and efecting prophylactic vaccine against HIV by allowing the development of a neutralizing antibody response.