Is Bone Loss Related to Immune Reconstitution in People on ART?

Immune system recovery and rising T-cell counts after starting antiretroviral therapy (ART) may contribute to bone loss among people with HIV, according to data from a small study presented at the 18th Conference on Retroviruses and Opportunistic Infections (CROI 2011) this month in Boston. A better understanding of early bone loss may allow for preventive therapies.

A growing body of evidence indicates that HIV positive people are prone to osteoporosis, or loss of bone mineral density, but it is not fully understood whether this is due to HIV infection itself, resulting inflammation, antiretroviral drugs, or some combination of these and possibly other factors.

Some studies find that bone loss occurs after starting HIV treatment, leading researchers to propose that it may be a side effect of ART in general or of specific drugs such as tenofovir (Viread, also in the Truvada and Atripla combination pills). Most research has looked at bone loss over time, however, and there is little data about changes immediately after starting therapy.

Ighovwerha Ofotokun from Emory University in Atlanta and colleagues conducted human and animal studies to evaluate when ART-related bone loss occurs and what might be contributing factors.

Since inflammation, T-cell activation, and altered cytokine production are known to contribute to bone loss in many diseases, the investigators prospectively analyzed changes in serum levels of RANKL (receptor activator of NF-kappa-B ligand), CTx (cross-linked C telopeptide of type I collagen), TNF-a (tumor necrosis factor-alpha), and osteocalcin at the start of treatment and after 2, 12, and 24 weeks on ART.

Activated T-cells produce RANKL, which stimulates osteoclasts, the cells that break down bone and promote resorption of its calcium and other minerals, the researchers noted as background; elevated levels are a marker of bone loss. A protein called osteoprotegerin binds to RANKL and thereby protects against bone loss. In contrast, osteocalcin is a protein produced by osteoblasts, the cells that build bone. CTx is a biomarker of collagen breakdown and bone resorption that is used to help diagnose osteoporosis.

The first analysis included 20 treatment-naive HIV positive men (average age 40 years) starting ART for the first time. After starting therapy they experienced an average CD4 T-cell gain of about 120 cells/mm3 and all but 1 achieved undetectable viral load by week 24.

The researchers then sought to evaluate the effects of T-cell reconstitution on bone loss in an animal model. They used TCRb knockout mice genetically engineered to lack specific T-cell receptors, and mimicked T-cell recovery on ART by introducing CD3 T-cells through adoptive transfer.

Results

• The HIV positive men experienced a "dramatic" early surge in bone resorption immediately after ART initiation, indicated by elevated levels of CTx, RANKL, and TNF-a.
• Bone loss was evident by week 2 (CTx elevation of about 50%), peaking at week 12 (CTx elevation of about 100%).
• Bone resorption then reached a plateau, but RANKL and TNF-a levels remained highly elevated at week 24.
• There was a compensatory increase in markers of bone formation -- indicated by about a 300% increase in osteocalcinat week 24 -- explaining why the men did not have extreme bone loss.
• In the mice, there was also a surge in bone resorption after introduction of T-cells.
• RANKL levels rose by 60% and CTx levels rose by about 150%.
• Bone resorption markers peaked around the same 12 week time point as they did in humans.
• By week 12 the mice showed reduced bone mineral density in the femur (thigh bone), tibia (shin bone), and lumbar spine.
• In contrast with the humans, however, the mice did not have compensatory bone formation, and in fact showed reduced osteocalcin levels.

The researchers concluded that ART-related bone loss begins earlier than previously suspected after treatment initiation, driven at least in part by T-cell activation and reconstitution.

The dramatic and unexpected early surge in bone resorption immediately after ART initiation was similar to changes seen in women at menopause, Ofotokun said. Although the data are not yet fully analyzed, he noted that there appears to be a correlation between extent of bone resorption and magnitude of CD4 cell gains -- an interesting finding in light of other recent research showing a link between bone loss and low nadir CD4 cell count.

The researchers plan to conduct further analyses to see whether use of tenofovir is associated with greater bone loss. Understanding early bone loss after beginning ART might lead to pre-emptive therapies to prevent or ameliorate it, Ofotokun suggested.

Investigator affiliation: Emory University, Atlanta, GA.

3/15/11

Reference

I Ofotokun, N Weitzmann, A Vunnava, et al. HAART-induced immune reconstitution: a driving force behind bone resorption in HIV/AIDS. 18th Conference on Retroviruses and Opportunistic Infections (CROI 2011). Boston. February 27-March 2, 2011. Abstract 78LB.