Is arteriosclerosis an autoimmune disease? Or the result of one?

It depends. There are several types of arteriosclerosis, and some types do not involve the immune system and some types are caused at least in part by the immune system. For example, atherosclerosis is a specific type of arteriosclerosis caused by the buildup of fat, cholesterol and calcium in arterial walls and its the "hardening of the arteries" that causes heart attacks. Inflammation (an immune response) makes this worse, but atherosclerosis is not caused by inflammation alone.

Arteriosclerosis in the brain is sometimes a result of ordinary atherosclerosis but it can also be caused by a process that is not well-understood. It can definitely occur as a secondary effect of autoimmune diseases such as multiple sclerosis, and it can also as a primary disease (as in Altzheimers).