Using molecular mimicry theory the answer would be
1) genetics- because the hla-b27 gene has similar structure to the collagen in the joints it means that when the immune system is exposed to klebsiella there can be an auto-immune response i.e. attacking klebsiella + collagen. without hla-b27 (or similar gene) there is no reason for the immune system to become confused.
2) immune system - because the immune system is involved in the disease process then anything that affects the immune system - colds, flu etc can have an effect on the disease.
3) diet - diet can increase or decrease the amount of klebsiella in the digestive tract making the auto-immune reaction stronger or weaker. diet can also influence intestinal permeability which affects how much of the bacteria in the gut the immune system can "see".
hope that helps
