Login failed for user 'IIS APPPOOL\DefaultAppPool'

Login failed for user 'IIS APPPOOL\DefaultAppPool'. When I change DefaultAppPool Identity To LocalSystem. when I get a new error 'Cannot Open DataBase'

Here is my connection string:

 <connectionStrings>
<add name="DBCS" connectionString="data source=.;database=Project;integrated security=SSPI" providerName="system.data.sqlclient"/>
</connectionStrings>

stack trace:

[SqlException (0x80131904): Login failed for user 'IIS APPPOOL\DefaultAppPool'.]System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions, SqlCredential credential, Object providerInfo, String newPassword, SecureString newSecurePassword, Boolean redirectedUserInstance, SqlConnectionString userConnectionOptions, SessionData reconnectSessionData, DbConnectionPool pool, String accessToken, Boolean applyTransientFaultHandling, SqlAuthenticationProviderManager sqlAuthProviderManager) +1431 System.Data.SqlClient.SqlConnectionFactory.CreateConnection(DbConnectionOptions options, DbConnectionPoolKey poolKey, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningConnection, DbConnectionOptions userOptions) +1085 System.Data.ProviderBase.DbConnectionFactory.CreatePooledConnection(DbConnectionPool pool, DbConnection owningObject, DbConnectionOptions options, DbConnectionPoolKey poolKey, DbConnectionOptions userOptions) +70 System.Data.ProviderBase.DbConnectionPool.CreateObject(DbConnection owningObject, DbConnectionOptions userOptions, DbConnectionInternal oldConnection) +964 System.Data.ProviderBase.DbConnectionPool.UserCreateRequest(DbConnection owningObject, DbConnectionOptions userOptions, DbConnectionInternal oldConnection) +109 System.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, UInt32 waitForMultipleObjectsTimeout, Boolean allowCreate, Boolean onlyOneCheckConnection, DbConnectionOptions userOptions, DbConnectionInternal& connection) +1529 System.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, TaskCompletionSource1 retry, DbConnectionOptions userOptions, DbConnectionInternal& connection) +156 System.Data.ProviderBase.DbConnectionFactory.TryGetConnection(DbConnection owningConnection, TaskCompletionSource1 retry, DbConnectionOptions userOptions, DbConnectionInternal oldConnection, DbConnectionInternal& connection) +258 System.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource1 retry, DbConnectionOptions userOptions) +312 System.Data.SqlClient.SqlConnection.TryOpenInner(TaskCompletionSource1 retry) +202 System.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource1 retry) +413 System.Data.SqlClient.SqlConnection.Open() +128 Foodora.com.WebForm3.CheckUser() in E:\Visual Studio 2012\Project\Project\SignUp.aspx.cs:49 Foodora.com.WebForm3.btnSubmit_Click(Object sender, EventArgs e) in E:\Visual Studio 2012\Project\Project\SignUp.aspx.cs:26 System.Web.UI.WebControls.Button.OnClick(EventArgs e) +11758480 System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +150 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +5028`