Immune changes in exposed, uninfected partners of people with HIV correlate with partner’s viral load

Changes from normal immune function in the exposed, uninfected sexual partners of people with HIV, is driven by continued exposure to their partner’s virus and is not innate, a Spanish study (Suy) has found.

The study found that the changes were closely correlated with the viral load in the HIV-positive partner, and that immune function in the uninfected partner reverted to a more typical picture after 90 days free from exposure to the partner’s HIV.

The immune changes observed were complex but basically involved an increasing level of differentiation in the immune system, with CD4 and CD8 cells being ‘pushed along’ faster in the development pathway so that more of them became memory cells, i.e. cells which are pre-sensitised to many different specific antigens, and fewer of them naïve cells, i.e. ones that have a slower response to a broad range of antigens.

However these responses were not HIV-specific; exposure to HIV seemed to result in a generally heightened state of ‘infection preparedness’ rather than a specific reaction to HIV.

The study’s findings are generally negative rather than positive; it did not report any single change that might confer immunity, temporary or permanent, specifically to HIV infection in the exposed, uninfected partners; but it did find that there was no link between genetic factors influencing the innate reactivity of their immune system to HIV.

Study method and population

The study looked at 21 monogamous heterosexual couples of discordant HIV status, average age 33, from one HIV clinic in Barcelona. In 16 couples, the woman was HIV-positive and the man HIV-negative; in the other five, the woman was HIV-negative and the man was HIV-positive. They had to have been having unprotected sex for a minimum of twelve months (average was three years, maximum 15 years); they also had to have had unprotected sex at least five times in the three months before trial entry (average number, eleven times) and at least once in the month before. None of the HIV-positive partners was on anti-HIV therapy.

All 42 individuals were tested for hepatitis B and C and for sexually transmitted infections (STIs) (none diagnosed) and received a number of immunological tests thus:

• CD4 and CD8 counts
• CCR5 and CXCR4 co-receptor expression on their CD4 and CD8 cells
• Presence of the Δ32 mutation in the CCR5 gene, which, if present in both the paternal and maternal genes, prevents CCR5 expression (and confers immunity to HIV)
• A variety of different subsets of CD4 and CD8 cells. These included tests to measure cell activation and maturation and whether cells were naïve (not exposed to antigens) or memory cells (exposed to antigens and ‘primed’ to recognise specific ones)
• Reactivity of cells to chemicals which induce cell division (a measure of cell reactivity)
• Reaction of CD4 and CD8 cells to specific antigens, i.e. short lengths of different proteins, from HIV, measured by interferon-γ production
• The presence of HIV-specific antibodies (IgA) in blood and, in the women, vaginal fluid.
• The HIV-positive partner was also tested for HIV viral load.

These immune responses of both partners were then compared with a control group of 22 HIV-negative monogamous heterosexual people who had unprotected sex.

The 21 serodiscordant couples were then asked to refrain from unprotected sex and use condoms for three months so that their immune responses could be measured again after a period of no exposure to HIV. Only ten couples actually managed this.

Results

The CD4 and CD8 counts of the HIV-exposed, HIV-negative partner in the serodiscordant couple were, as to be expected, more similar to the HIV-negative low risk controls (CD4s in HIV-positives, 384; in exposed negatives, 830; in unexposed negatives, 965). CD8 counts were higher in the HIV-positive people (777 as opposed to 480 in exposed seronegative and 598 in unexposed seronegative respectively).

However when it came to subsets of CD4 cells, the immune status of the HIV-exposed, seronegative partners was more similar to their seropositive partner than to unexposed HIV-negative people.

In particular, the number and percentage of certain subsets both of CD4 memory cells, and of cells that were in transition to becoming memory cells, were higher in the serodiscordant couples (indeed, they were nearly identical) than they were in unexposed HIV-negative people.

Similarly the percentage and numbers of naïve cells were lower, and more similar to each other, in both serodiscordant partners than they were in unexposed HIV-negative people.

In the case of CD8 cells, the picture was less clear-cut, but a similar phenomenon was observed; the immune responses of the exposed, HIV-negative partners tended more to mirror those of their positive partners than those of other HIV-negative people.

In some cases the immune systems of exposed seronegative people seemed ‘sluggish’, less reactive than those of unexposed people. They responded no more quickly to chemicals inducing cell division than did their HIV-positive partners, and less quickly than unexposed people. Similarly, CD38 (a general maker of cellular activation, which is dampened down in HIV-positive people but goes high during progression to AIDS) was lower and rather more similar in the serodiscordant couples than it was to HIV-negative people.

Finally and strikingly, both HIV-positive people and their exposed partners had higher levels of the CCR5 co-receptor on their CD4 cells than did HIV-negative people. In theory this should mean that their cells were more, rather than less, vulnerable to attack by HIV.

Were they immune to attack because they had developed specific responses to their partner’s HIV, as if vaccinated by viral antigens? This does not appear to be the case since one of the striking findings of the study was that hardly any of the exposed seronegative partners had immune responses - either cellular or humoral (antibody) – which were specific for HIV.

When exposed to HIV antigens, the CD8 cells of four exposed negative partners reacted weakly to them, but no one produced a significant CD4 cell response. When this was measured in a different way, by measuring interferon-γ production, only two exposed negative partners produced weak responses compared to13 of the positive partners.

Eight out of 21 exposed negative partners had very low levels of anti-HIV IgA antibodies in their blood, whereas all HIV- positive partners had antibody levels at least 27 times higher than this. This is to be expected, as if the exposed partners had had similar levels of IgA, they would have tested HIV-positive. Similarly, all the HIV-positive women had IgA antibodies to HIV in their vaginal secretions and none of the HIV-negative women.

There was a strong and significant correlation between the positive partner’s HIV viral load and the percentage of CD4 and CD8 cells in the negative partner that were non-HIV-specific memory cells. There was a 100% (twofold) increase in the proportion of cells that were memory cells for each one log (tenfold) increase in the positive partner’s viral load, and a 25% increase in CD8 memory cells per one log increase in viral load.

There was an inverse correlation between the positive partner’s viral load and the proliferative (dividing) capacity of all subsets of the negative partner’s T-cells. There was a 50% decrease in the ‘stimulation index’ of these T-cells in the negative partner per one log increase in their positive partner’s viral load.

When the ten couples who had managed to maintain 100% condom use for 90 days had their immunological status measured again after this time, the immune status of the negative partner had become more like that of an unexposed HIV-negative person, with fewer memory and more naïve cells, and an increased proliferative capacity.

Their CCR5 levels stayed high, but levels of the other cellular HIV co-receptor, CXCR4, decreased.

Finally, five of the exposed seronegative partners and three of the HIV-positive partners turned out to be ‘heterozygous’ for the Δ32 mutation in their CCR5 gene, which means they carried one out of two copies of the mutated CCR5 gene. In the past Δ32 heterozygosity has been hypothesised as offering some degree of resistance to progression to AIDS in people who are infected. However the immune responses of the exposed seronegative partners carrying Δ32 did not differ in the slightest from the other participants, and the three HIV-positive partners (all women) who carried Δ32 were amongst the individuals with the highest viral loads and, in two cases, lowest CD4 counts.

Conclusions

This study tells us that significant immune changes appear to be induced in HIV-negative people who are exposed (regularly) to their partner’s HIV, and that these changes correlate with the amount of HIV ‘challenge’ they get. These changes ‘push’ their immune system into a state that in some way more nearly mirrors that of a reasonably healthy HIV-positive person that it does that of a low risk unexposed negative person.

It also shows that these responses are not permanent but are ‘topped up’ by HIV exposure.

Finally, it also shows that genetic and innate resistance to HIV infection, at least in this small sample, does not influence these immune responses.

However its other findings are mainly negative. Exposure to HIV does not appear to generate a very specific response to HIV; instead it produces a broad response whose clinical significance is unknown, and we cannot tell if these immune changes are protective or are just a marker for HIV exposure, and cannot tell if they are responsible for keeping these people uninfected for periods of time ranging from one to 15 years, despite consistent unprotected sex with partners with significant HIV viral loads, or if some other protective factor is at work.

It is worth saying that some other studies have reported different findings. For instance, some researchers have found specific CD8 responses to HIV antigens in the blood of exposed seronegative people (Kebba), albeit weak ones, and another study (Kaul) found CD8 responses to HIV antigens in the vaginal secretions of eleven out of 16 frequently exposed but uninfected Nairobi sex workers.

Others have found that some “exposed uninfected” partners of people with HIV actually are infected – but at an extremely low level. For instance Zhu found that two out of 94 HIV-exposed partners of gay men, when tested with a hyper-sensitive PCR viral load assay, did have HIV in their blood but at a level a thousand times lower than that detectable by conventional viral load tests – 0.05 copies. The infection appears to have been contained so well that it did not provoke an antibody response and they therefore tested HIV-negative.