A slow release of antigen (depot effect) is supposed to provide continual stimulation to the immune system which is expected to amplify and sustain immune responses. The antigen is present locally for a longer duration, so APCs reaching the site at later time points can pick it up and present it to T cells. This leads to a stronger/longer immune response. Since all the antigen is not released into the bloodstream at the same time, it also reduces the occurrence of severe adverse events (SAEs).Adverse events are usually mild to moderate and are local reactions limited to the injection site.

Depot effect is usually talked about in the context of intramuscular injections. Your antigen-adjuvant goes into the muscle and the insoluble adjuvant keeps the antigen at the injection site for dendritic cells to come by and pick it up. The APCs migrate through the bloodstream to the local lymph nodes where the antigen is then presented to T cells. Alum is the most commonly used adjuvant that has a depot effect. So the slow release is not happening in the blood, it happens at the injection site.

As far as I know, no one has actually quantified the velocity/speed of release. And I don't think there is any way you can precesely control the speed of antigen release from your antigen-adjuvant complex. There is an abundance of literature/experience out there showing that adjuvants boost immune responses (innate and adaptive) and that alum is one of the most immunogenic adjuvants.

There is no perfect way to answer the Q you have. If you change the formulation (i.e. add an adjuvant), then the depot effect follows. So you can't delineate whether the immune response is from the addition of the adjuvant or from the depot effect. And usually, the increased T cell response you observe is the result of the adjuvant activating innate immune signaling pathways (IFN, inflammasome, TLR), the slow release of antigens and the sustained activation of APCs and the increased expression of cytokines and inflammatory markers.

Anyway, if you want to follow biodistribution of vaccines once injected to look at slow vs. fast release, you could do bioimaging. A couple of papers are linked below for further reading. You could find a lot more online.

Article Unique depot formed by an oil based vaccine facilitates acti...

Article In Vivo Electroporation Enhances the Immunogenicity of Hepat...

Julie answered most of your question.

I would just stress that the depot effect adjuvants, especially the oil/water emulsions CANNOT be injected IV. You will cause a LOT of inflammation and tissue death. And you may even block the capillaries.

The slow release of antigens is supposed to mimic what happens during a natrual infection when it is cleared over time.

Yes, completely agree with Vinu's statements.

The depot effect is only supposed to mimic the continued release of antigens during a natural infection. There are SO many other mechanisms and pathways that are activated during natural infection, which is why many adjuvant research is now focused on immune response modulation. Trying to get the immune cells to develop the correct adapative response in a way that it is protective.

Adjuvants are generally added to inactivated antigens that are administered either intramuscularly, subcutaneously or intranasally. As indicated above by Julie, the depot effect occurs at the site of injection, not in the blood stream. If you are developing a human vaccine, the adjuvant that is generally used is an alum or aluminum hydroxide. However, it is not the best adjuvant for animal vaccines. One adjuvant does not work best for all antigens.

With oil-based adjuvants particle size is important to define the time of residence of the depot effect. Generally, the larger the particle size, the longer the residence time (depot effect). On the down side, the larger the particle size, the greater the local reactivity. Water/Oil/Water and Water/Oil adjuvants have the largest particle sizes (in the range of 50 to 100 microns. Oil/Water adjuvants vary in particle size. Some, such as the adjuvants produced by MVP adjuvants (www.mvpadjuvants.com) are controlled to a particle size between 1 and 6 microns.

Intranasally-administered adjuvants will have a submicron particle size in order to allow absorption and stimulation of the mucosal immune system (mostly IgA).