The present study will address the potential effectiveness of add-on pyridostigmine (90mg, once per day, per oris) on CD4+ T cell counts, CD4+/CD8+ ratio, as well as ex-vivo markers of T cell phenotype and activity. The study is designed as a 24-week crossover study where patients will start a 12-week of pyridostigmine or placebo, and then crossing-over for an aditional 12 weeks (placebo-to-pyridostigmine, and pyrodistigmine-to-placebo).