Since you are using ristocetin and von Willebrand factor, you are looking at agglutination. Ristocetin will induce a conformational change in VWF which allows it to interact with platelet GPIb. This interaction will consequently result in a platelet-VWF agglutinate. 

Dear Frederik,

Thanks for your message.

Do you have any reference that supports your answer? 

Best,

Some interesting references supporting my answer:

Berndt MC, Ward CM, Booth WJ, Castaldi PA, Mazurov AV, Andrews RK. Identification of aspartic acid 514 through glutamic acid 542 as a glycoprotein Ib-IX complex receptor recognition sequence in von Willebrand factor. Mechanism of modulation of von Willebrand factor by ristocetin and botrocetin. Biochemistry. 1992;31:11144–11151.

Dong JF, Berndt MC, Schade A, McIntire LV, Andrews RK, López JA. Ristocetin-dependent, but not botrocetin-dependent, binding of von Willebrand factor to the platelet glycoprotein Ib-IX-V complex corre- lates with shear-dependent interactions. Blood. 2001;97:162–168.

Kang M, Wilson L, Kermode JC. Evidence from limited proteolysis of a ristocetin-induced conformational change in human von Willebrand factor that promotes its binding to platelet glycoprotein Ib-IX-V. Blood Cells Mol Dis. 2008;40:433–443.

When you are making an aggretation test with platelet rich plasm (platelets metabolicaly actives), in presence of ristocetin, the firts wave is the result of VWF and IbIXV interaction. That produce activation, IIbIIIa conformational change, and finally a second wave originated by IIaIIIb-fibrinogen interaction.

Aglutinattion wil be produce when fixed platelets are used in ristocetin cofactor assay.

 Von Willebrand factor in the platelet adhesiveness linked to endothelial colagen  is mediated by glycoprotein Ib/V/IX, a structurally unique receptor complex expressed in     Megacariocytes and platelets, initiating the multiples pathways of plateklet activation, VWF coulkd participate in the initials reactións of platelet aggregation but is the fibrinogen with the integrin IIb-IIia the principal components of this mechanism.

Platelet Activation and Atherothrombosis

Giovanni Davì, M.D., and Carlo Patrono, M.D.

N Engl J Med 2007; 357:2482-2494

FVW interacts in its D 'domain with FVIII by maintaining it in an active state, but inhibiting its activity on thrombin and prolonging its half-life in plasma. Another mode of action of FVW is competing with the phospholipid binding sites of FVIII and preventing the coagulation factor from being degraded by serine protease activity, including activated protein C. The activity of VWF in the process of platelet plug formation is important, since the platelets adhere to cells of the Subendothelial matrix by the interaction of FVW with collagen I and III via its A3 domain (E1673-G1874) and its A1 (E1260 and G1479) domain with the platelet glycoprotein GPIbα (GPIb-IX-V complex subunit). In addition, FVW binds to sulfated glycolipids that serve as accessory molecules in their interaction with platelets in the A1 domain. In platelet adhesion, FVW multimers are more involved because they have multiple anchor sites to glycoprotein receptors. Platelets are activated before vascular damage by different mechanisms that increase transduction signals, which leads to the redistribution of cytoskeletal proteins, to the exposure and activation of GP IIb / IIIa receptors and to the P-selectin translocation of The alpha granules to the platelet surface. This allows aggregation of adjacent platelets, inter alia, by P-selectin from the surface of platelets to their receptors, ligand glycoproteins (PSGL-1). GPIa / IIa (α2β1 integrin) is another important receptor of Platelets and their interaction with FVW allows an increase in their affinity for collagen. When the GPIa / IIa and the GPIb-IX-V interface complex bind to FVW, the collagen is exposed to P-selectin to repair blood vessel rupture. Intracellularly, platelet activation by VWF leads to activation of the enzyme phosphatidylinositol-3-kinase which stimulates the synthesis of phosphatidylinositol 3, 4, 5-triphosphate (PIP3)

If you are using washed platelets (as was stated in the question) and you merely add ristocetin and purified VWF, you are in fact looking at agglutination. Ristocetin will bind to VWF and because of this, the VWF in solution will be able to interact with platelet GPIb. Normally in solution this would not occur, since VWF circulates in a folded (inactive) conformation. The contribution of platelet activation and subsequent aggregation is less 'strong' in this set up. We in fact recently demonstrated that we could dissolve these agglutinates by cleaving VWF that is holding together the platelets. Denorme F et al. Blood 2016: https://doi.org/10.1182/blood-2015-08-662650

Hi, ricosetin added to VWF will induce aggregation of platelets by causing transformational changes.

If you know the mechanism of binding of von Willebrand factor to factor VIII protein which then helps to bring about adherence of platelets to this complex then it would be best termed as aggregation.

In a first step, the interaction of VWF with platelet GPIb is crucial to adhesion (reversible). After, Collagen and GPVI allow togheter with VWF and GPIb some intracellular changes and consequent activation of platelets: the aggregation is the result of Fibrinogen and GPIIbIIIa binding, but also from VWF and FVIII action in these estimulated platelets and endothelial cells.

In a strict sense, Ristocetin only induces agglutination; Fibrinogen is necessary to platelet aggregation.

Your experiment is very smart and I understood the occurrence of an increase of platelet aggregation with a corresponding lower light transmission. So, you confirmed platelet agglutination induced by Ristocetin! Agglutination is the fact that you proved.

Sorry for the answer delay, but I only read your message today in a web search related to this matter.

doi: 10.3389/fimmu.2019.02884 (main scientific reference).