Received 6/18/97, Accepted 7/22/97

We now wish to present twelve steps on how we think molecular immunity against retroviruses actually works, then we will describe data which we feel support this hypothesis:

i) Initial exposure: Upon initial exposure to a retrovirus, the host organism requires a brief lag period to marshal its molecular responses.

ii) Initial viral activity: During the lag period, the retrovirus actively replicates or infects some cells then becomes dormant, depending on the nature of the retrovirus.

iii) Lag period of host: During the lag period, myriad intracellular defense mechanisms become activated, and these responses have evolved over time to block the replication of the retrovirus at every important step of its life cycle.

iv) Virus-specific messenger molecule: Following the lag-period, which seems to last 2-5 days in healthy individuals, retrovirus-specific messenger molecules are produced by the host. These molecules relay pathologic information to uninfected cells, probably in some sort of genetic fashion, and seem to be persistent for long-term immunity.

v) Cascade of molecular responses: The messenger molecules are highly specific to the particular retrovirus in question, and they activate a cascade of events in the uninfected cells, which allow these cells to arm themselves against infection by that particular retrovirus.

vi) Immunity against genetically-closely-related viruses: Although this immunity is highly specific to the retrovirus which initially infects the host, molecular immunity also inhibits the infection by retroviruses that are closely related genetically.

vii) Late evolutionary, and ontogenic development of molecular immunity: The development of molecular immunity seems to have occurred relatively late in evolution, and consequently, it matures relatively late in the ontogenic development of the organism. For humans, molecular immunity seems to develop between the ages of 0.5-3 years. Prior to this maturation, infection of a young host with a retrovirus of relatively low pathogenicity or a low-dose infection with a pathogenic retrovirus may prove uncontrollable or fatal.

viii) CD8+ lymphocytes are basis of response: A subset of CD8+ lymphocytes initiate the molecular response to retroviruses, and these cells also dispatch the messenger molecules to various cells in the organism, including CD4+ T-cells, monocyte/macrophage cells, and cells of the CNS (i.e. neurons, astrocytes, oligodendrocytes). Upon exposure to the messenger signals, the recipient cells synthesize their own anti-retroviral agents, which are designed to inhibit the early steps in the life cycle of retrovirus, such as RT, RNase H and integrase activities. Interruption or dysfunction of the primary responder cells—namely specific CD8+ lymphocytes—can result in unchecked replication of the retrovirus .

ix) Cofactors can compromise immunity: Protective molecular immunity against a specific strain of retrovirus and genetically-closely-related viruses would be life long unless untoward events or cofactors adversely affect the intracellular mechanisms of protection or the subsets of lymphocytes that are involved in molecular immunity (particularly CD8+ T-cells). These cofactors are especially important during the initial infection and lag period, at which time they can determine the future course of infection. "Untoward events or cofactors" are of several types, including exposure to low-dose radiation, UV-light, cyclosporins, cyclophosphamide, or protein-synthesis inhibitors. Another type of cofactor is co-infection with another pathogenic virus or exposure to specific viral products—these other viruses need not necessarily be retroviruses. A third type of cofactor is temporary immunoincompetence due to abuse of certain substances, such as alcohol, cocaine or even certain antiviral drugs—these latter cofactors may be of particular significance during the initial exposure and lag periods. No doubt, there are also yet unknown cofactors, which interfere or inactivate the molecular immunity pathways, and which will render the host susceptible to acute infection of retroviruses. (118-125).

x) Low-dose inoculation can provide molecular immunity: A low-dose inoculation of a pathogenic retrovirus will result in the development of protective molecular immunity against the specific retrovirus or against genetically-closely-related retroviruses. Provided cofactors do interfere excessively in the initial immunologic response. This is akin to a subclinical infection, where low-dose exposure to a pathogen results in the development of protective immunity without overt signs or symptoms of the disease. Similarly, exposure to a non-pathogenic live retrovirus provides an immunity against genetically-closely-related pathogenic retroviruses (54-58). The degree of success in developing molecular protection is directly proportional to the degree of genetic relatedness of challenging retroviruses, and inversely proportional to the degree of pathogenicity of the inoculating retrovirus.

xi) Active replication necessary for effective vaccine: The activation of molecular immunity requires exposure of primary target cells to live retrovirus. A heat-inactivated, formalin-fixed whole virus, or virion subunits produced by recombinant vectors, or peptides and subunits produced by bacterial or baculovirus expression systems, or any other form of a vaccine where the virus is unable to complete a single cycle of growth, would not be able to bring molecular immunity to its full potential and thus would not protect the host. In other words, the very mechanisms of retroviral infection—not the antigenic nature of the virus itself—elicit the molecular response that leads to protective immunity (54-58, 66).

xii) Possible HIV-1 vaccines: The hypothetical vaccines that should produce protective immunity against HIV-1 are the following live-virus vaccines (in descending order of desirability): 1) an engineered replication defective HIV-1 virus (i.e env-defective virus), 2) very low doses of a genetically-closely-related, non-pathogenic lentivirus, such as SIVcpz, 3) low doses of a non-pathogenic HIV-1 virus, such as a particular nef-defective strain, or 4) a controlled, very low-dose inoculation with fully pathogenic HIV-1 (obviously this method would require very stringent conditions).

In summary, a new hypothesis has been forwarded which provides numerous examples, both from experimental data and experiments of nature that actual protection against retroviruses is from a third, yet unexplored, form of immunity- "molecular immunity". Also, there are ample evidence to show that traditional form of immunities- cell-mediated and humoral- do not play any significant role in protection against retroviruses.