**Study Overview:** This trial is testing if doxycycline, a common antibiotic, can slow down emphysema, a lung disease, in people with HIV, a virus that attacks the immune system. Emphysema makes it hard to breathe because it damages the air sacs in the lungs. Participants will either take doxycycline or a placebo (a pill with no medicine) for **72 weeks**. **Key Points:** - The study will include **250 people** living with HIV and emphysema. - During the trial, participants will take **two pills a day** and attend several visits for tests and check-ups. - The study will look at the safety of doxycycline, and whether it causes antibiotic-resistant infections. Participants must be 30 years or older, have stable HIV treatment, and be able to take oral medication. They cannot join if they have had certain lung surgeries, infections, or recent use of some medications. The study measures how well doxycycline works by checking lung function and overall health.