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In spite of repeated exposures to HIV, some individuals remain seronegative and apparently uninfected. A variety of mechanisms potentially able to confer resistance to HIV infection, including cell-mediated and (unconventional) humoral immune responses, as well as mutations affecting receptors for virus entry have been considered and analysed. In this article, we want to discuss recent reports on specific immune responses and genetic factors potentially involved in mechanisms of protection, and to present some of our data relative to a cohort of people sexually exposed to HIV-1, but persistently seronegative. These EU (exposed uninfected) individuals can be distinguished from "normal" unexposed controls on the basis of significantly increased frequencies of a number of immunological parameters that might be considered "unconventional" correlates of HIV infection/protection. However, EU individuals are highly heterogeneous since the various unconventional immune responses considered can be present in all possible combinations. Aim of future research will be to ascertain the role of such immune responses in the maintenance of the protection state, or their secondary nature as signals of a particular kind of infection.

Since the clinical earliest descriptions of patients with acquired immune deficiency syndrome (AIDS) it has been very clear that a profound state of immunologic dysfunction was the underlying cause of the emergence of life-threatening opportunistic infections and tumors. In addition to the progressive loss of CD4 "helper" T lymphocytes, a profound defect in interleukin-2 (IL-2) production was recognized as a major pathogenic component of the new disease. For these reasons, attempts to administer IL-2 to individuals infected with the human immunodeficiency virus (HIV), the causative agent of AIDS, have been made since the mid eighties, however with little success. On the other hand, the propensity of HIV to replicate in activated lymphocytes and macrophages, under the influence of the cytokine network, has represented, and in part still does, a major hurdle for the rationale of administering IL-2 or other cytokines to HIV-infected individuals. Major steps forward towards an understanding of the role of multiple components of the immune system, coupled with a potentially successful protocol of IL-2 administration in vivo, resulting in the stable uprising of circulating CD4+ T cells, shed an optimistic light on the possibility to achieve a substantial immune reconstitution in HIV-infected individuals, thus preventing the onset of AIDS.