the invention provides vectors , attenuated pathogens , compositions , methods , and kits for use in preventing or treating infection by an infectious pathogen , especially chlamydia trachomatis . the vectors comprise the plasmid encoded ppg genes from chlamydia , ppg1 , ppg2 , ppg3 , ppg5 , ppg6 , ppg7 and / or ppg8 , but lack ppg4 , a regulator of virulence associated genes . the application also provides attenuated pathogens , especially chlamydia , which are cured of their plasmid and have additional mutations to improve the attenuation , especially mutations in the ct135 gene or in the tryptophan operon . uses of said nucleic acids and attenuated pathogens for inducing or modulating an immune response in a subject , especially for prevention or treatment of infections , are proposed .